Various labor intensive steroid hormone regimens have been used to control the equine estrous cycle; however, results are extremely variable, due in part to a prolonged follicular phase. Recently, ultrasound-guided transvaginal follicle aspiration was used as a method of follicle ablation for synchronizing follicular wave emergence and ovulation in cattle at random stages of the estrous cycle (Therio 42:895-907, 1994). The present study was done to determine the efficacy of the ablation procedure as an alternative to steroid treatment for ovarian synchronization in mares. Regardless of stage of the estrous cycle, horse mares were assigned to 1 of 3 groups: 1) Control (no follicle ablation, n=10), 2) Ablation (n=14) and 3) Ablation + hCG (n=6). Human CG (1,500 IU, im) was given when the largest follicle was first detected at a diameter of ≥35 mm to enhance ovulation synchrony during the follicular phase. Prior to follicle ablation, mares were sedated with detomidine hydrochloride (0.02 mg/kg, iv) and caudal epidural anesthesia was induced with 6 ml of 2% lidocaine. A 5.0 MHz convex-array transducer was used transvaginally to guide a 17-gauge aspiration needle into the antrum of all follicles ≥10 mm in diameter. Follicle ablation was defined by collapse of the follicle following evacuation of follicular contents. On the fourth day after start of the experiment (ablation), mares in the control and ablation groups were given PGF (250 μg cloprostenol, im) in the morning and again in the evening to ensure luteal regression. Ovarian changes were monitored daily by transrectal ultrasonography from the start of the experiment until the day of ovulation. Follicular wave emergence (first detection of follicles ≥10 mm in diameter) combined for both ablation groups occurred (mean±SEM), 2.4±0.5 d post-ablation; largest follicle diameter at first detection was 10.9±1.1 mm. The diameter of the ovulatory follicle 1 d prior to ovulation was not significantly different among groups (control, 40.7±1.5 mm; ablation, 38.8±1.1 mm; ablation + hCG, 37.7±1.1 mm).The mean interval to ovulation after PGF and its associated variance were not different between the 2 ablation groups; therefore, the data were combined. Although the interval to ovulation was shorter (P=0.08) in the control group than in the ablation groups, the degree of variation was reduced (P=0.09) in follicle-ablated mares. The inequality of variance between control and ablated-mares was attributed to a higher degree of ovulation synchrony in the ablation group. Fifteen of 20 ablated mares ovulated 7 to 12 d following PGF, whereas only 4 of 10 control mares ovulated during the same time period (P=0.06). Of the 6 mares in the ablation + hCG group, 4 (67%) ovulated 7 (n=3) and 8 (n=1) d after PGF. The remaining 2 mares ovulated 12 and 13 d after PGF. Unexpectedly, 5 of 14 mares in the ablation alone group had double ovulations compared to 1 of 10 control mares (P=0.15). In summary, ultrasound-guided transvaginal follicle ablation done at random during the equine estrous cycle resulted in follicular wave emergence within 3 d post-ablation and in significantly greater (75%) ovulation synchrony compared to controls post-PGF. Moreover, results suggested that ovulation synchrony in follicle-ablated mares can be further tightened by incorporation of hCG treatment during the follicular phase. Surprisingly, the ablation procedure resulted in a relatively high double ovulation rate (36%), a phenomenon that warrants further investigation.