Walrus in US zoos have a very low reproductive rate of 11 births in 80 years, and little is known about Pacific walrus (Odobenus rosmarus divergens) reproductive biology. To address this, we initiated a program in which detailed biological data were recorded on captive walrus. As part of a 7-year study, 1 male and 1 female 16-year-old captive Pacific walrus were carefully monitored with weekly serum hormone analysis, daily glans penis smears for spermatozoa, and abdominal ultrasound for pregnancy. The female ovulated once annually from late December through midJanuary and then exhibited 9 months of sustained elevated progesterone. This nonconceptive estrous cycle profile is consistent with reports from wild walrus females. In contrast, the male's seasonal rut routinely occurred in late February through May with a serum testosterone peak in March. This profile differed from the reported adult male cycle in wild walrus of November through March. During the period of the female's ovulation, the male had nadir testosterone levels and was consistently azoospermic. Likewise, during the male's spermatogenic rut in the spring, the female was anovulatory with elevated progesterone. On this basis, the male was treated for 14 weeks with human chorionic gonadotropin (hCG) in an attempt to increase testosterone levels in synchrony with the female's annual ovulation. The treatment successfully induced rut characterized by sustained elevated serum testosterone levels and production of spermatozoa. The male and female successfully bred, and the female became pregnant. Upon discontinuation of hCG treatment, the male resumed baseline testosterone levels. We theorize that the lack of synchronization of rut and ovulatory cycles is a primary reason for reproductive failure in these captive walrus.