Technical Abstract: Although kept in zoos for over 29 years, the crocodile monitor has proven difficult to breed in captivity. Despite observations of courtship and copulation, most zoos report that females lay infertile eggs. The crocodile monitor is a high priority for conservation management; however, limited information is available about the reproductive biology of this species. The aim of this study was to characterize physiological aspects of reproductive cycles with the intent of developing management strategies to improve captive propagation. Through a collaborative effort among 5 institutions, the endocrine profiles and gonad dynamics of 22 crocodile monitors were evaluated during a 2-year period. In female crocodile monitors, average estrogen peaks (3,402 pg/ml) were 10-fold higher than baseline levels; while testosterone levels in females remained low (10'28 ng/ml) throughout the study period. Ultrasound data indicated that estrogen spikes occurred during initial follicular growth one month before the onset of vitellogenesis. Interestingly, males demonstrated both testosterone (422 +/- 55.3 ng/ml) and estrogen (449 +/- 192.8 pg/ml) peaks, with estrogen peaks approximating the rise in plasma testosterone. Although males and females exhibited multiple steroid hormone peaks annually, the timing of reproductive cycles varied from year to year for continuously-paired and singly-housed monitors. The timing of yearly reproductive cycles was more similar for intermittently paired monitors; however, asynchrony between intermittently paired males and females also was evident. Frequent evaluation of female reproductive cycles, using the noninvasive methods outlined in this study, may be useful in determining optimal timing of male/female introductions for successful reproduction.