Timing of Sexual Behavior in the Female Rat

Virgin female rats were tested to determine the beginning of sustained sexual behavior with reference to vaginal cycling. Records of vaginal smear sequences established that the subjects cycled every 4 days. Coitus was first observed in 29% of the females in the early afternoon of proestrus. Eighty-eight per cent were fully receptive by 1930 on the day of proestrus. It is suggested that receptivity may occur before systemic progesterone reaches peak concentrations. The data demonstrate that coitus occurs in close proximity to the critical period for ovulatory release of LH on the day of proestrus. (Endocrinology86: 1181, 1970)