Urinary corticoids were measured in humans (n = 9) in frequently collected urine samples taken during a 48 hr period, and in captive western lowland gorillas (n = 5) and free-ranging mountain gorillas (n = 3) from samples taken from 0700 to 1800 hr. In each study, the highest concentrations occurred in the morning hours, then declined gradually, reaching the lowest levels in the afternoon to evening. These data show that a similar diurnal pattern of corticoid excretion does occur in these species. We suggest that if single-sample urine collection for determination of hyperadrenal activity is to be used, urine is best collected during hours of low corticoid excretion. © 1994 Wiley-Liss, Inc.