AbstractIn chimpanzees, ovulation time can be estimated in relation to the time of sexual swelling regression, itself a periovulatory parameter. Correlations of steroid hormone and gonadotropin levels, endometrial morphology, basal temperature, and ovarian morphology consistently suggest a close temporal correlation between ovulation and the last days of maximal sexual swelling.In the gorilla, LH elevations occur during the one to four days of labial swelling, suggesting that ovulation is restricted to this easily detectable phase of the menstrual cycle.The menstrual cycle of the orang‐utan lacks an externally visible periovulatory indicator, such as the sexual swelling, for correlation with ovulation time. Possibly a sharply defined period of female sexual proceptivity could provide a usable periovulatory criterion applicable to captive orang‐ut