Stress is known to alter a variety of biological processes, including behavior and reproduction. It is therefore important to understand the stress levels of animals in captivity, especially those for whom captive breeding is a priority, such as the okapi. Levels of stress hormones can be measured from samples collected noninvasively, such as urine or feces, which are preferable with nondomestic species for whom drawing blood might in itself be a considerable stressor. To understand the excretion of cortisol in urine in the okapi, four (1.3) animals were subject to three injections: saline, 200 IU of an adrenocorticotropic hormone (ACTH) analogue, and 300 IU of the analogue. Their 24-hr urinary corticosteroid levels were compared with 4 baseline days. Injection with the ACTH analogue significantly increased the urinary corticosteroid levels compared with saline injections and baseline. Eight (3.5) okapi were then observed for 24 hr per day for 5 days to determine their normal patterns of corticosteroid production. The mean corticosteroid levels varied significantly by individual. A significant circadian pattern in urinary corticosteroid was apparent independent of individual or gender, with cortisol rising during the daylight hours and decreasing again at night. Zoo Biol 27:381-393, 2008. (c) 2008 Wiley-Liss, Inc.