Six, mature female capuchins, C. apella, were used to study occurrence of bleeding, the length and phases of the menstrual cycle, and the gestation period. During a 3 mo. period the menstrual cycle was determined to be 21 days, including an average of 4 days in which bleeding was detectable. 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 1 aborted. The other 2 remained barren. The average gestation period for the 3 normal pregnancies was 153 days.