diff --git a/NEWS.md b/NEWS.md
index 369d9338..e7558031 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -28,6 +28,8 @@ the dosing including dose amount and route.
   `summary()` (#477)
 
 * New post-processing functions to normalize PKNCA result parameters based on any column in PKNCAconc data.frame (`normalize_by_col()`) or by using a custom normalization table (`normalize()`)
+* New excretion rate parameters: `ermax` (Maximum excretion rate), `ertmax` (Midpoint time
+  of maximum excretion rate) and `ertlst` (Time of last excretion rate measurement) (#433)
 
 # PKNCA 0.12.1
 
diff --git a/R/001-add.interval.col.R b/R/001-add.interval.col.R
index 076fee9e..908d3192 100644
--- a/R/001-add.interval.col.R
+++ b/R/001-add.interval.col.R
@@ -114,9 +114,9 @@ add.interval.col <- function(name,
       choices=c(
         "unitless", "fraction", "%", "count",
         "time", "inverse_time",
-        "amount", "amount_dose",
+        "amount", "amount_dose", "amount_time",
         "conc", "conc_dosenorm",
-        "dose",
+        "dose", 
         "volume",
         "auc", "aumc",
         "auc_dosenorm", "aumc_dosenorm",
diff --git a/R/pk.calc.urine.R b/R/pk.calc.urine.R
index aeb472a2..d919170f 100644
--- a/R/pk.calc.urine.R
+++ b/R/pk.calc.urine.R
@@ -33,29 +33,12 @@ PKNCA.set.summary(
 #' @seealso [pk.calc.clr()], [pk.calc.fe()]
 #' @export
 pk.calc.ae <- function(conc, volume, check=TRUE) {
-  mask_missing_conc <- is.na(conc)
-  mask_missing_vol <- is.na(volume)
-  mask_missing_both <- mask_missing_conc & mask_missing_vol
-  mask_missing_conc <- mask_missing_conc & !mask_missing_both
-  mask_missing_vol <- mask_missing_vol & !mask_missing_both
-  message_both <- message_conc <- message_vol <- NA_character_
-  if (all(mask_missing_both)) {
-    message_both <- "All concentrations and volumes are missing"
-  } else if (any(mask_missing_both)) {
-    message_both <- sprintf("%g of %g concentrations and volumes are missing", sum(mask_missing_both), length(conc))
-  }
-  if (all(mask_missing_conc)) {
-    message_conc <- "All concentrations are missing"
-  } else if (any(mask_missing_conc)) {
-    message_conc <- sprintf("%g of %g concentrations are missing", sum(mask_missing_conc), length(conc))
-  }
-  if (all(mask_missing_vol)) {
-    message_vol <- "All volumes are missing"
-  } else if (any(mask_missing_vol)) {
-    message_vol <- sprintf("%g of %g volumes are missing", sum(mask_missing_vol), length(conc))
-  }
-  message_all <- stats::na.omit(c(message_both, message_conc, message_vol))
-  ret <- sum(conc*volume)
+  # Generate combined missing-data messages for conc/volume using helper
+  message_all <- generate_missing_messages(conc, volume,
+                                           name_a = "concentrations",
+                                           name_b = "volumes")
+
+  ret <- sum(conc * volume)
   if (length(message_all) != 0) {
     message <- paste(message_all, collapse = "; ")
     ret <- structure(ret, exclude = message)
@@ -155,3 +138,189 @@ PKNCA.set.summary(
   point=business.geomean,
   spread=business.geocv
 )
+
+#' Calculate the midpoint collection time of the last measurable excretion rate
+#'
+#' @param conc The concentration in the excreta (e.g., urine or feces)
+#' @param volume The volume (or mass) of the sample
+#' @param time The starting time of the collection interval
+#' @param duration.conc The duration of the collection interval
+#' @param check Should the concentration and time data be checked?
+#' @return The midpoint collection time of the last measurable excretion rate, or NA/0 if not available
+#' @export
+pk.calc.ertlst <- function(conc, volume, time, duration.conc, check = TRUE) {
+
+  # Generate messages about missing concentrations/volumes
+  message_all <- generate_missing_messages(conc, volume,
+                                           name_a = "concentrations",
+                                           name_b = "volumes")
+
+  if (all(is.na(conc))) {
+    ret <- NA_real_
+  } else if (all(conc %in% c(0, NA))) {
+    ret <- 0
+  } else {
+      midtime <- time + duration.conc / 2
+    midtime <- time + duration.conc / 2
+    ret <- max(midtime[!(conc %in% c(NA, 0))])
+  }
+
+  if (length(message_all) != 0) {
+    message <- paste(message_all, collapse = "; ")
+    ret <- structure(ret, exclude = message)
+  }
+  ret
+}
+
+# Add the column to the interval specification
+add.interval.col("ertlst",
+                 FUN="pk.calc.ertlst",
+                 unit_type="time",
+                 pretty_name="Tlast excretion rate",
+                 desc="The midpoint collection time of the last measurable excretion rate (typically in urine or feces)")
+
+PKNCA.set.summary(
+  name="ertlst",
+  description="median and range",
+  point=business.median,
+  spread=business.range
+)
+
+#' Calculate the maximum excretion rate
+#'
+#' @param conc The concentration in the excreta (e.g., urine or feces)
+#' @param volume The volume (or mass) of the sample
+#' @param time The starting time of the collection interval
+#' @param duration.conc The duration of the collection interval
+#' @param check Should the concentration data be checked?
+#' @return The maximum excretion rate, or NA if not available
+#' @export
+pk.calc.ermax <- function(conc, volume, time, duration.conc, check = TRUE) {
+
+  # Generate messages about missing concentrations/volumes
+  message_all <- generate_missing_messages(conc, volume,
+                                           name_a = "concentrations",
+                                           name_b = "volumes")
+
+  if (length(conc) == 0 || all(is.na(conc))) {
+    ret <- NA
+  } else {
+    er <- conc * volume / duration.conc
+    ret <- max(er, na.rm=TRUE)
+  }
+
+  if (length(message_all) != 0) {
+    message <- paste(message_all, collapse = "; ")
+    ret <- structure(ret, exclude = message)
+  }
+  ret
+}
+
+add.interval.col("ermax",
+                 FUN="pk.calc.ermax",
+                 unit_type="amount_time",
+                 pretty_name="Maximum excretion rate",
+                 desc="The maximum excretion rate (typically in urine or feces)")
+
+PKNCA.set.summary(
+  name="ermax",
+  description="geometric mean and geometric coefficient of variation",
+  point=business.geomean,
+  spread=business.geocv
+)
+
+#' Calculate the midpoint collection time of the maximum excretion rate
+#'
+#' @param conc The concentration in the excreta (e.g., urine or feces)
+#' @param volume The volume (or mass) of the sample
+#' @param time The starting time of the collection interval
+#' @param duration.conc The duration of the collection interval
+#' @param check Should the concentration and time data be checked?
+#' @param first.tmax If TRUE, return the first time of maximum excretion rate; otherwise, return the last
+#' @return The midpoint collection time of the maximum excretion rate, or NA if not available
+#' @export
+pk.calc.ertmax <- function(conc, volume, time, duration.conc, check = TRUE, first.tmax = NULL) {
+
+  # Generate messages about missing concentrations/volumes
+  message_all <- generate_missing_messages(conc, volume,
+                                           name_a = "concentrations",
+                                           name_b = "volumes")
+
+  if (length(conc) == 0 || all(conc %in% c(NA, 0))) {
+    ret <- NA
+  } else {
+    er <- conc * volume / duration.conc
+    ermax <- pk.calc.ermax(conc, volume, time, duration.conc, check = FALSE)
+    midtime <- time + duration.conc / 2
+    ret <- midtime[er %in% ermax]
+
+    if (first.tmax) {
+      ret <- ret[1]
+    } else {
+      ret <- ret[length(ret)]
+    }
+  }
+
+  if (length(message_all) != 0) {
+    message <- paste(message_all, collapse = "; ")
+    ret <- structure(ret, exclude = message)
+  }
+  ret
+}
+
+add.interval.col("ertmax",
+                 FUN="pk.calc.ertmax",
+                 unit_type="time",
+                 pretty_name="Tmax excretion rate",
+                 desc="The midpoint collection time of the maximum excretion rate (typically in urine or feces)")
+
+PKNCA.set.summary(
+  name="ertmax",
+  description="median and range",
+  point=business.median,
+  spread=business.range
+)
+
+
+
+# Helper to generate missing-data checking messages for paired vectors
+#
+# This function accepts two columns/vectors (for example, concentrations
+# and volumes). It computes missingness internally and produces a character
+# vector of human-readable messages describing the missingness that matches
+# the style used in the package (used previously in `pk.calc.ae`).
+generate_missing_messages <- function(a, b,
+                    name_a = deparse(substitute(a)),
+                    name_b = deparse(substitute(b))) {
+
+  mask_a <- is.na(a)
+  mask_b <- is.na(b)
+  
+  mask_both <- mask_a & mask_b
+  mask_a_only <- mask_a & !mask_both
+  mask_b_only <- mask_b & !mask_both
+  
+  msg_both <- msg_a <- msg_b <- NA_character_
+  n <- length(mask_a)
+  
+  if (all(mask_both)) {
+    msg_both <- sprintf("All %s and %s are missing", name_a, name_b)
+  } else if (any(mask_both)) {
+    msg_both <- sprintf("%g of %g %s and %s are missing", sum(mask_both), n, name_a, name_b)
+  }
+  
+  if (all(mask_a_only)) {
+    msg_a <- sprintf("All %s are missing", name_a)
+  } else if (any(mask_a_only)) {
+    msg_a <- sprintf("%g of %g %s are missing", sum(mask_a_only), n, name_a)
+  }
+  
+  if (all(mask_b_only)) {
+    msg_b <- sprintf("All %s are missing", name_b)
+  } else if (any(mask_b_only)) {
+    msg_b <- sprintf("%g of %g %s are missing", sum(mask_b_only), n, name_b)
+  }
+  
+  # Return non-NA messages
+  stats::na.omit(c(msg_both, msg_a, msg_b))
+}
diff --git a/R/unit-support.R b/R/unit-support.R
index 46cba380..74982d4a 100644
--- a/R/unit-support.R
+++ b/R/unit-support.R
@@ -77,6 +77,7 @@ pknca_units_table <- function(concu, doseu, amountu, timeu,
       pknca_units_table_dose(doseu = doseu),
       pknca_units_table_conc_dose(concu=concu, doseu=doseu),
       pknca_units_table_conc_time(concu=concu, timeu=timeu),
+      pknca_units_table_time_amount(timeu=timeu, amountu=amountu),
       pknca_units_table_conc_time_dose(concu=concu, timeu=timeu, doseu=doseu),
       pknca_units_table_conc_time_amount(concu=concu, timeu=timeu, amountu=amountu),
       pknca_units_table_conc_time_amount_dose(concu=concu, timeu=timeu, amountu=amountu, doseu=doseu)
@@ -367,6 +368,19 @@ pknca_units_table_conc_time_amount <- function(concu, timeu, amountu) {
   )
 }
 
+pknca_units_table_time_amount <- function(timeu, amountu) {
+  if (useless(timeu) || useless(amountu)) {
+    time_amount <- NA_character_
+  } else {
+    time_amount <- sprintf("%s*%s", timeu, amountu)
+  }
+  data.frame(
+    PPORRESU = time_amount,
+    PPTESTCD = pknca_find_units_param(unit_type = "amount_time"),
+    stringsAsFactors = FALSE
+  )
+}
+
 pknca_units_table_conc_time_amount_dose <- function(concu, timeu, amountu, doseu) {
   if (useless(concu) || useless(timeu) || useless(amountu) || useless(doseu)) {
     renal_clearance_dosenorm <- NA_character_
diff --git a/tests/testthat/test-pk.calc.urine.R b/tests/testthat/test-pk.calc.urine.R
index 3c066005..392fae19 100644
--- a/tests/testthat/test-pk.calc.urine.R
+++ b/tests/testthat/test-pk.calc.urine.R
@@ -60,3 +60,75 @@ test_that("pk.calc.fe", {
                0.3,
                info="fe is calculated correctly with both vector/scalar")
 })
+
+test_that("pk.calc.ertlst", {
+  # All NA
+  expect_equal(
+    pk.calc.ertlst(conc = c(NA, NA), volume = c(1, 1), time = c(0, 1), duration.conc = c(1, 1)),
+    structure(NA_real_, exclude = "All concentrations are missing")
+  )
+  expect_equal(
+    pk.calc.ertlst(conc = c(NA, NA), volume = c(NA, NA), time = c(0, 1), duration.conc = c(1, 1)),
+    structure(NA_real_, exclude = "All concentrations and volumes are missing")
+  )
+  # All 0 or NA
+  expect_equal(
+    pk.calc.ertlst(conc = c(0, NA), volume = c(1, 1), time = c(0, 1), duration.conc = c(1, 1)),
+    structure(0, exclude = "1 of 2 concentrations are missing")
+  )
+  # Normal case
+  expect_equal(
+    pk.calc.ertlst(conc = c(1, 2, 0), volume = c(1, 1, 1), time = c(0, 1, 2), duration.conc = c(1, 1, 1)),
+    max(c(0, 1) + 1/2)
+  )
+})
+
+test_that("pk.calc.ermax", {
+  # All NA
+  expect_equal(
+    pk.calc.ermax(conc = c(NA, NA), volume = c(1, 1), time = c(0, 1), duration.conc = c(1, 1)),
+    structure(NA, exclude = "All concentrations are missing")
+  )
+  # Normal case
+  expect_equal(
+    pk.calc.ermax(conc = c(1, 2, 3), volume = c(2, 2, 2), time = c(0, 1, 2), duration.conc = c(2, 2, 2)),
+    max(c(1, 2, 3) * 2 / 2)
+  )
+})
+
+test_that("pk.calc.ertmax", {
+  # All NA or 0
+  expect_equal(
+    pk.calc.ertmax(conc = c(NA, 0), volume = c(1, 1), time = c(0, 1), duration.conc = c(1, 1)),
+    structure(NA, exclude = "1 of 2 concentrations are missing")
+  )
+  # Normal case, last tmax
+  expect_equal(
+    pk.calc.ertmax(conc = c(1, 3, 2), volume = c(2, 2, 2), time = c(0, 1, 2), duration.conc = c(2, 2, 2), first.tmax = FALSE),
+    (1 + 2/2)
+  )
+  # Normal case, first tmax
+  expect_equal(
+    pk.calc.ertmax(conc = c(1, 3, 2), volume = c(2, 2, 2), time = c(0, 1, 2), duration.conc = c(2, 2, 2), first.tmax = TRUE),
+    (1 + 2/2)
+  )
+  # Multiple maxima
+  expect_equal(
+    pk.calc.ertmax(conc = c(1, 3, 3), volume = c(2, 2, 2), time = c(0, 1, 2), duration.conc = c(2, 2, 2), first.tmax = TRUE),
+    (1 + 2/2)
+  )
+  expect_equal(
+    pk.calc.ertmax(conc = c(1, 3, 3), volume = c(2, 2, 2), time = c(0, 1, 2), duration.conc = c(2, 2, 2), first.tmax = FALSE),
+    (2 + 2/2)
+  )
+})
+
+test_that("generate_missing_messages", {
+  # Ensure that the deparse(substitute()) methods work
+  conc <- NA_real_
+  volume <- NA_real_
+  expect_equal(
+    as.character(generate_missing_messages(conc, volume)),
+    "All conc and volume are missing"
+  )
+})