A breeding colony of owl monkeys (Aotus trivirgatus) established in 1972 produced 35 live infants during the first 3 years from an average daily population of 30 monogamous pairs. Initial productivity was poor, but once the breeding pairs were set up by karyotypes, reproductive performance improved. No seasonal influence on births was observed and most births occurred during daylight hours. Uterine size during gestation ranged from about 1 cm in diameter at 30 days after conception to 7--8 cm (transverse diameter) x 18--20 cm (longitudinal diameter) at 115 days after conception. The fetal head was palpable at 85 days. A gestation period of 120-140 days was estimated from extrapolation of uterine palpation data and interbirth intervals. Vaginal swabs collected from six females during two separate 1-month periods did not consistently reveal estrous cycles. An analysis of owl monkey milk showed it to contain 88.2% water, 2.16% crude protein, 2.09% fat, 7.52% lactose, and 0.36% ash.