Five female beagles were examined periodically throughout their second biannual cycle of proestrus, estrus, metestrus, and anestrus for fluctuations in several physiological and behavioral charateristics. Physiological measures included plasma levels of estradiol and progesterone as well as the vulvar swelling and vaginal bleeding that begin with proestrus and continue in estrus. Behavioral tests revealed changes in female receptivity, attractivity, and proceptivity. Estradiol increased prior to the onset of proestrus and declined during estrus. Progesterone levels were low during most of proestrus, increased from the beginning to the end of estrus, and remained elevated during the first month or more of metestrus. While they were in proestrous females were not receptive but they and their vaginal secretions were highly attractive to males. Attractivity remained high throughout estrus and declined abruptly in the next 24 hr. Receptivity increased over the first 3 days of estrus and continued at a high level until the last 3 days during which it decreased slightly, and then dropped nearly to zero within the next 24 hr. “Sexual reflexes” of the vulva, tail, and hindquarters followed the same course of changes described for attractivity. Proceptive behavior, including seeking proximity to caged males and display of solicitation responses during mating tests was characteristic of females during proestrus and estrus but uncommon or absent in other phases of the cycle. It is hypothesized that in the natural cycle, attractivity and proceptivity develop during proestrus as a consequence of rapidly increasing secretion of estrogen. The onset of receptivity is due to synergistic action of estrogen and progesterone. Estrogen secreted during proestrus primes the system, and progesterone secreted just before and during estrus activates it. Termination of receptivity is thought to be due to the marked decline in estrogen, to inhibition by high concentrations of progesterone, or to a combination of these factors.
