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We evaluated a GnRH contraceptive vaccine in penned feral swine that were captured throughout Florida. In March 2002, pigs (~7-30kg) were segregated by sex and assigned to two GnRH treatments, given as a single IM injection in the rump using AdjuVac adjuvant. Ten males and 9 females received 1000μg GnRH-KLH; 10 males and 9 females received 2000μg GnRHKLH. Untreated males and females served as controls. In mid-June 2002, blood was taken and males and females were combined into two large breeding pens. Animals were euthanized in December and blood and tissue samples were taken. Reduced testicular and ovarian size, and serum testosterone and progesterone were evident in GnRH-treated animals and corresponded to increased antibody titers to GnRH. The most effective GnRH vaccine dose for females was 2000μg; for males it was 1000μg. The 2000μg GnRH vaccine prevented pregnancy in 100% of the females at slaughter and 90% of the females during the 36-week study. For females receiving the 1000μg GnRH vaccine, 78% were not pregnant at slaughter, but only 44% were not pregnant during the entire study. We conclude that the single-shot GnRH vaccine is highly effective for contraception of feral female swine for up to 36 weeks.