On Dec. 11, 1974, 15 seasonally anestrous mares were assigned at random to 1 of 3 experimental groups: outdoor-control, indoor-control, or indoor light-treated (a 16 h photoperiod). This experiment was terminated on April 21, 1975. The 5 mares in the indoor light-treated group ovulated 59.0 .+-. 6.9 days later, which was 74 days earlier (P < 0.01) than 2 of the 5 outdoor-controls (the other 3 ovulated after April 21 during a subsequent experiment) and 50 days earlier (P < 0.05) than the indoor-controls. Durations of the 1st estrus for the 3 groups of mares were 13.3 .+-. 3.6, 8.4 .+-. 2.0, and 6.0 .+-. 1.0 days for the indoor light-treated, indoor-control, and outdoor-control groups, respectively. The indoor light-treated mares averaged 4.2 estrous cycles before April 21, the indoor-control mares averaged 1.4 estrous cycles, and 2 of 5 outdoor-control mares ovulated 1 time during the experiment. The peripheral blood luteinizing hormone (LH), estradiol and progesterone concentrations were minimal during winter anestrous. The hormone changes normally associated with estrous cycle activity in mares, maximal estradiol and luteinizing hormone concentrations near ovulation and maximal progesterone concentration during diestrus, were observed in all mares beginning at the 1st estrus. Hair loss was observed earlier in the light-treated mares, than in either of the other groups. A 16 h photoperiod initiated in early Dec. for anestrous brood mares caused endocrinologically normal estrous cycles to begin within 2 mo. This may allow breeding and foaling considerably earlier than normally expected.