Six, mature female capuchins, Cebus apella, were used to study occurrence of bleeding, the length and phases of the menstrual cycle, and the gestation period. During a 3-month period the menstrual cycle was determined to be 21 days, including an average of 4 days in which bleeding was detectable. It was found that only proestrus and estrus could be reliably identified during the menstrual cycle by the use of vaginal smears. Each female was housed with a male for a sufficient time to allow insemination. Four became pregnant. Three had term pregnancies and one aborted. The other two remained barren. The average gestation period for the three normal pregnancies was 153 days.