The effects of a single or double regimen of exogenous progesterone and estradiol-17beta (P/E, total dose 300 mg P/20 mg E) were investigated in 50 postparturient Quarter Horse mares. In Trial 1, at 1 and 24 h after foaling, mares were injected with progesterone (150 mg) and estradiol-17beta (10 mg) (n = 7) or 0.9% NaCl (control, n = 13). In Trial 2, within 12 h after foaling, mares were injected with progesterone (300 mg) and estradiol-17beta (20 mg) (n = 13) or 0.9% NaCl (control, n = 17). Mares were examined daily by palpation per rectum and transrectal ultrasonography to determine the day of ovulation. The largest cross sectional diameters of each uterine horn and uterine body were measured ultrasonographically on Day 15 postpartum. Mean uterine diameters did not differ between treatment groups (P > 0.05) in Trial 1, Trial 2 or for combined data for both Trials 1 and 2. For mares bred on the first postpartum estrus pregnancy rates did not differ (P > 0.05) between treatment groups (16/18, 89%) and controls (22/30, 81%) nor was there a difference in mean day to first postpartum ovulation (P > 0.05) between treated and control groups in Trial 1, Trial 2 or Trials 1 and 2 combined. However, fewer (P < 0.05) total P/E treated mares (0/20) ovulated prior to Day 10 postpartum than did control mares (6/30). Variance in days to ovulation was lower (P < 0.05) for P/E treated mares (var = 3.73 days) than for control mares (var = 7.64 days) for data combined from Trials 1 and 2.