Estrus recurs at intervals averaging 1655 days (variability 6.1%), is most frequent at night (62-68%), and lasts for approximately 8.22 hrs. (range 1-18). Proestrus behavior starts 10-25 hrs. before sexual receptivity or estrus, and lasts until 1 hr. or so after the first copulatory reflex. Proestrus behavior is marked by increased activity of a form similar to that of the normal confined [female]; this includes 65-85% of the cases. A particular animal is consistent in its behavior, while considerable variability exists between animals. The beginning and duration of estrus is most easily and reliably detected by the copulatory reflex or lordosis. The intensity of estrus is greatest at the beginning, and gradually diminishes. Ovulation occurs within 11/2 hr. after the end of estrus. In some animals at least copulation tends to shorten estrus.