This study tested several hypotheses regarding the functions of urine washing (UW) in a captive group of brown capuchin monkeys (Cebus apella), using observational and experimental methods. Observational data obtained over a 16 month period revealed no correlation between rates of UW and age or aggression. Rates of UW did not vary when the subjects were exposed to their mirror images or when their hands were soiled. Transferring the group to an unfamiliar environment had no effect on UW frequencies. Climatological factors most clearly influenced rate of UW: The behavior occurred more frequently in high temperatures and during sunny periods, indicating a possible role of UW in thermoregulation.