Homogeneous Silastic^® rods containing ethinylestradiol (EE) (1.5 or 4 g), estradiol-17β (E) (4 g) or progesterone (P) (6 g) were implanted into feral mares (Equus caballus) between 4-and 10-yr-old. Six treatment groups (≥10 mares/group) of non-pregnant mares received 36 g P and 12 g E (P E), 36 g P and 8 g EE (P HEE), 1.5 g EE (LEE), 3 g EE (MEE), 8 g EE (HEE) or control-implanted mares (CI). CI received implants containing no steroid. Two groups of pregnant mares received P HEE or HEE. Stallions were placed with the mares 15 to 26 mo after implanting. Blood was collected biweekly for up to 28 mo after implanting and serum analyzed for P by radioimmunoassay. A single P value ≥2.5 ng/ml indicated ovulation and 2 consecutive values ≥2.5 ng/ml indicated pregnancy. Serum from blood collected before and at 4, 12, 24, 50, 64 and 89 wk after implanting was analyzed for EE concentrations. All animals pregnant at the time of contraceptive placement delivered normal foals. Contraceptive efficacy for groups LEE, MEE, HEE and P HEE were 75, 75, 100, and 100%, respectively after two breeding seasons. Suppression of ovulation appeared to be inversely related to the concentration of EE used in the implant. The percent of animals ovulating after 2 yr of contraception in each group was 100, 100, 88, 62, 20, and 12 for groups CI, P E, LEE, MEE, HEE and P HEE, respectively. The pregnancy rate for the same groups was 100, 78, 25, 25, 0 and 0%, respectively. Contraceptive efficacy was followed for 3 yr in one group, P HEE, and was 88%. Pregnancy rates for groups P E and CI after 3 yr was 78 and 82%, respectively. Our data demonstrate effective contraception of feral mares for up to 36 mo without compromising a pregnancy in effect at the time of implanting. Calculating the decline in EE concentrations to 150% of pre-implantation concentrations, these data suggest an effective contraceptive life of approximately 16, 26, and 48 to 60 mo for LEE, MEE and HEE implants, respectively. Mechanisms that appear to be involved in contraceptive efficacy include preventing ovulation at higher concentrations of steroids and either suppressing ovulation or implantation at lower concentrations of steroid.