The succession of vaginal and behavioral changes was followed throughout the estrous cycle of ten continuously cycling female rats. Interruption of cycles by pregnancy or pseudopregnancy was prevented by the use of the vaginal mask. The behavioral indices were Lordosis Quotient, a measure of sexual responsiveness, and Rejection Quotient, a measure of tolerance by the female of mounting by the male. Before the onset of lordosis, which took place during proestrus, females rejected males on many attempted mounts, although rejection decreased rapidly with rise in frequency of lordosis. Full receptivity was displayed for 6 to 9 hours, but females began to reject the males while still displaying high levels of lordosis. The total period of 19 hours over which lordosis is shown was divided into pre-heat, heat, and post-heat periods. The succession of vaginal and behavioral changes was followed throughout the estrous cycle of ten continuously cycling female rats. Interruption of cycles by pregnancy or pseudopregnancy was prevented by the use of the vaginal mask. The behavioral indices were Lordosis Quotient, a measure of sexual responsiveness, and Rejection Quotient, a measure of tolerance by the female of mounting by the male. Before the onset of lordosis, which took place during proestrus, females rejected males on many attempted mounts, although rejection decreased rapidly with rise in frequency of lordosis. Full receptivity was displayed for 6 to 9 hours, but females began to reject the males while still displaying high levels of lordosis. The total period of 19 hours over which lordosis is shown was divided into pre-heat, heat, and post-heat periods.