Substantial progress in understanding the reproductive physiology of the four rhinoceros species in captivity, White rhinoceros Ceratotherium simum, Black rhinoceros Diceros bicornis, Indian or Greater one-horned rhinoceros Rhinoceros unicornis and Suma-tran rhinoceros Dicerorhinus sumatrensis, has been achieved over the last two decades largely owing to advances in endocrine and ultrasound techniques, and improved methods of semen collection. Primarily through the non-invasive monitoring of hormone metabolites, scientists have studied the reproductive cycle, investigated the effects of season and potential stressors, and diagnosed pregnancy in each species. By employing the powerful tool of ultrasonography, more detailed and accurate information regarding reproductive function has been revealed. Important findings include anovula-tory cycles, induced ovulation, reproductive tract pathologies, early pregnancy diagnosis and early embryonic loss. Interest in the reproductive physiology of c? rhinoceros has lagged behind that of ♀♀ but in recent years data on sperm and semen characteristics, reproductive glands and hormone production have been collected. While the reproductive physiology of each rhinoceros species is interesting in its own right, perhaps most fascinating is the significant inter-specific variation in reproductive traits within this taxon.