The contraceptive effect of active immunization against GnRH was evaluated in beef heifers. Crossbred heifers were randomized by breed and weight (initial weight = 227.3 +/- 1.2 kg) and assigned to one of three treatment groups. Animals in Group IB (n = 48) were actively immunized against GnRH; heifers in Groups NB (n = 48) and NN (n = 49) did not receive the anti-GnRH vaccine. Sixteen weeks after primary immunization, bulls of proven fertility were introduced into pens containing Groups IB and NB. Bulls were maintained with heifers for 2 mo. Heifers in group NN were not intentionally exposed to fertile males. At the end of the breeding period, heifers received Synovex H implants and entered a commercial feedlot. Heifers were slaughtered after 116 d of feedlot confinement. Anti-GnRH titer was evident in all heifers (48 of 48) immunized against GnRH. Gravid uteri were present at slaughter in 40 of 48 (83.3%) NB heifers. In contrast, only four (8.3%) IB and two (4.1%) NN heifers carried gravid uteri at slaughter. Although ADG did not differ between groups during feedlot confinement, ADG during the breeding period was higher (P < .05) in IB heifers than in NN control animals. Dressing percentage and longissimus muscle area were decreased (P < .05) and marbling and quality grade were increased (P < .05) in NB heifers compared with NN control heifers. Carcass traits of IB heifers were intermediate between those of the NB and NN groups. Taken together, these data indicate that active immunization against GnRH reduces the fertility of terminal heifers. These observations suggest that immunoneutralization of GnRH may be an effective management tool that will reduce the incidence of unintended pregnancy in heifers destined for feedlots.