Four- and 5-day cyclic mice housed with and without males, respectively, were autopsied at 10 a.m. on proestrus (P) or estrus (E) after no treatment, or after ovariectomy (Ovax) or sham Ovax at 10 a.m. or 4 p.m. on one of the preceding 3 days. Results indicate the ovaries must be in situ between 4 p.m. the day before P and 10 a.m. the day of P for a normal proestrous uterine weight increase to occur in both 4- and 5-day cyclic mice. For estrous cornification to occur, ovaries must be in situ between 10 a.m. and 4 p.m. on diestrus (D) in 4-day cyclic mice, and between 4 p.m. on D-I and 10 a.m. on D-II in 5-day cyclic mice. Sham Ovax tended to inhibit ovulation in both 4- and 5-day cyclic mice. The effects of both Ovax and sham Ovax on proestrous and estrous pituitary LH values were variable. Timing of the feedback relationship between ovary and hypothalamus-pituitary in mice may be imprecise, leading to easy disruption of the estrous cycle in this species.