The population of great hornbills (Buceros bicornis) in the United States is rapidly aging, and captive breeding efforts have not met population managers' expectations for a sustainable captive group. Little is known about the reproductive physiology of these birds. This study reports the first data on the re-productive endocrinology of the great hornbill. The hormone profiles of the only pair of these birds that hatched a chick in the 1999–2000 breeding season are compared to the profiles of six other pairs of hornbills, from different institutions in the United States, that did not reproduce successfully that season. The study investigates the estradiol, corticosterone, and testosterone profiles of these seven pairs of birds, establishing a base of knowledge from which endocrine data may be used to improve the success of captive breeding programs. The estradiol profiles from this study indicate a difference in hormonal patterns between laying and non-laying female great hornbills. Egg-laying females had significantly higher estradiol concentrations during the breeding season than the non-laying females (P<0.003). Testosterone concentrations of the males were not significantly different between the mates of egg-laying and non-egg-laying females. The corticosterone concentrations tended to be lower in the females that laid eggs vs. the non-egg-laying group. The males of the egg-laying pairs showed a significantly lower (P<0.036) corticosterone concentration than the non-egg-laying male pairs. This, combined with the extremely low corticosterone levels (compared to the other birds in the study) of the pair of hornbills that hatched a chick in the 1999–2000 breeding season, suggests that adrenal activity may play a role in the reproductive failure of some captive great hornbills. Zoo Biol 22:135–145, 2003. © 2003 Wiley-Liss, Inc.