Thirty-four intact female rats were observed at hourly intervals in standardized tests designed to reveal the presence and intensity of sexual receptivity. The onset of this condition was related to the artificial light-dark cycle, with a majority of females becoming receptive a few hours before or shortly after the beginning of the dark phase. Disappearance of receptivity was less closely related to conditions of illumination. The median duration of the receptive period was 19 hours. As measured by a specially devised "Sensitivity Index", the degree of sexual responsiveness increases sharply during the early stages of the receptive period, and then undergoes a more gradual decline. The duration of the female's mating response (lordosis) depends in part upon the nature of the male's mating reaction. Mounting without intromission evokes a very brief reaction, whereas a response involving ejaculation by the male elicits a protracted response on the part of the receptive female. This seems to be independent of the stage of receptivity but to bear some relation to the individual level of over-all sexual responsiveness of the particular female in question.