SUMMARY Length of estrous cycle and ovarian activity in the nonpregnant mare is related to season of the year and should not be characterized independently of season. Studies at the University of California have indicated that 75 to 85% of mares develop signs of estrus and ovulate from April through October, but these responses to endocrine activity are characteristic of only 20 to 25% of mares in January and February. During the period February through June, length of the estrous cycle was 20 to 21 days, estrus lasted 5½ days, and ovulation occurred 24 to 48 hours before the end of estrus. Average lifespan of the corpus luteum was 12 to 13 days, with regression beginning about 3 days before the onset of estrus. Cyclic ovarian activity may be interrupted due to failure of the corpus luteum to regress. The same type of syndrome has been observed in hysterectomized mares, i.e., removal of the uterus during the luteal phase of the cycle resulted in prolongation of luteal function. In both naturally occurring and experimentally induced situations, progestins suppressed manifestation of estrus but did not suppress follicle growth or, in some instances, ovulation. The best judgment of when to breed the mare is derived from the psychic behavior of the mare when teased by the stallion, the condition of the cervix when palpated and viewed with the speculum, and follicle development on the ovary as determined by palpation.