SummaryThe events of the perineal swelling cycle in Macaca nemestrina have been correlated with the optimal time for conception. The mean cycle length of the animals was 32·8 days with the follicular and luteal phases of the cycle 17·6-19·2 and 13·6-15·2 days respectively. The time of ovulation, relative to the tumescence peak, ranged from I to 13 days. The optimal time for mating, based on the ratio between day of breeding and cycle length was 0·46, a value comparable with similar calculations for other macaque species.