Two trials were conducted to examine reproductive function and feedlot performance by heifers after active immunization against GnRH. In trial 1, heifers were not immunized or were immunized with one of three doses of a GnRH-KLH (keyhole limpet hemocyanin) conjugate in Freund's complete adjuvant. Antibodies against GnRH were not detectable in non-immunized heifers (n = 9). However, antibodies against GnRH were noted in all immunized animals (n = 30) within 8 wk of primary immunization; anti-GnRH antibody concentrations were at a maximum 16 to 20 wk after immunization. This increased anti-GnRH titer was associated with a decreased serum concentration of progesterone. Ovarian and uterine weight and tissue concentrations of LH and GnRH receptor were reduced (P less than .05) by immunoneutralization of GnRH. Similarly, immunization against GnRH reduced (P less than .05) weight gain during feedlot confinement. In trial 2, feedlot performance after insertion of anabolic steroid implants (Synovex H) was evaluated in non-immunized heifers (n = 15), heifers actively immunized against GnRH-KLH (n = 15) or KLH alone (n = 15), or non-immunized heifers treated with melengestrol acetate (MGA; n = 15). Serum concentrations of progesterone were depressed in anti-GnRH and MGA-fed groups, but ovarian and uterine weights were depressed (P less than .05) only in heifers immunized against GnRH. Total weight gain and gain during the final 4 wk of confinement did not differ (P greater than .05) among groups with steroid implants. The GnRH-KLH conjugate is an effective immunogen in heifers, leading to suppression of reproductive activity. The depression of weight gain that attends development of anti-GnRH titers may be reversed by use of implants that contain anabolic steroids.