The reproductive biology of the slow loris (Nycticebus coucang) is poorly documented because of infrequent captive breeding success and the absence of field studies of this species. Reproductive data were collected from a breeding colony of slow lorises held at the Duke University Primate Center for the past 10 years. Nineteen infants were born, with a sex ratio of 1:1 and a neonatal mortality rate of 15.8%. In all cases, litter size was one. Females born in the colony copulated for the first time between 18 and 24 months of age. A male that reached sexual maturity in the colony sired his first offspring at the age of 17 months. Estrous cycles ranged in duration from 29-45 days, with copulations usually occurring for 1 day of estrus. Gestation length averaged 192.2 days. Although a postpartum estrus was observed in three cases of infant death, no conceptions resulted. Lactation lasted approximately 6 months. A clear birth peak was observed, with 12 out of 19 births occurring in March, April, and May. The comparatively low basal metabolic rate of this species may account for the unusually low reproductive rate of the slow loris in comparison with other prosimian primates.