Two pony mares were immunized against recombinant porcine inhibin alpha subunit three times with 39 day intervals. Clinical findings and endocrinological changes before immunization were taken as the control. The first significant rise in the anti-inhibin titre (P<0.05) in the circulation was found 27 days after the first injection. Maximum binding activity was reached by the 12th day after the second booster dose. The number of small, medium and large sized follicles had increased significantly compared to before immunization (11.75 +/- 4.30, 2.75 +/- 0.69 and 2.51 +/- 0.63 vs 6.50 +/- 1.43, 1.83 +/- 0.44 and 1.33 +/- 0.38, respectively), but the ovulation rate remained unchanged after immunization. The average plasma concentration of FSH and estradiol-17beta during the estrous cycle increased significantly (P<0.05) after immunization. These results suggest that immunization against inhibin is a useful tool to increase the number of ovarian follicles during the estrous cycle of pony mares. Moreover, the present study supported the concept that inhibin plays a major role in the control of follicular growth through its inhibitory effect on FSH secretion synergistically with steroid hormones.