Urinary testosterone and cortisol concentrations were quantified in a large number of samples (>500) collected from wild male chimpanzees (n=11) over the course of 1 year. For both steroids, urinary concentrations were higher and more variable in the morning than in the afternoon. Urinary creatinine levels showed no such diurnal pattern. These patterns are consistent with studies of steroid excretion in humans and gorillas. This study emphasizes the importance of considering time of day as a confounding variable in field studies of primate endocrine function. It also suggests that if a small number of samples are to be used to characterize an individual's basal steroid levels, afternoon samples may be preferable because they show less intra-individual variability. Am. J. Primatol. 60:161–166, 2003 © 2003 Wiley-Liss, Inc.