Feral horse population growth rates as high as 25% are of concern to those responsible for managing range lands as well as conservation groups. Current methods to control these populations include adoption and long-term holding, which are both costly and controversial. Porcine zona pellucida (pZP) immunocontraception may have the greatest potential to control fertility because it has proven to be effective in other studies and vaccines are easy and safe to administer. One pZP vaccine formulation, SpayVac® (ImmunoVaccine Technologies, Inc., Halifax, NS, Canada), has demonstrated single-dose, multi-year contraceptive efficacy in other wildlife species, which would make it both practical and economical for field application. Over a 7-month period during the breeding season, we assessed the effect on ovarian activity of 2 formulations of SpayVac, 1 non-aqueous with modified Freund's adjuvant (MFA) and the other, an aqueous emulsion with MFA, compared to controls (n = 7 per group). Comparative reproductive parameters included serum concentrations of progesterone (P4) determined by enzyme-linked immunosorbent assay (ELISA), ovarian activity assessed by transrectal ultrasound and palpation, as well as gross and histological examination of ovaries upon necropsy (n = 9 or 3 mares from each group) or after ovariectomy (n = 12 or 4 mares from each group). We determined serum antibody titers using ELISA. Mean serum concentrations of P4 were less in the non-aqueous MFA treatment group compared to control mares (P < 0.025). Ovaries collected from control mares weighed more (P = 0.002) and had greater variation (P = 0.003) than those from either vaccinated group. Both treatment groups also had smaller ovaries and fewer follicles compared to controls (P < 0.001). Three to 4 months after vaccination, 93% of SpayVac-injected mares ceased cycling; whereas all control mares continued to cycle throughout the study. Relatively constant antibody titers were reached by week 6 post-vaccination, although we found appreciable variation within treatment groups, especially 4–8 weeks post-vaccination. Based on our study, the SpayVac formulations impair ovarian function but do not affect other major organ systems, and could provide a safe and effective immunocontraceptive option for mares. Additional research to elucidate the vaccine's mechanism of action, actual contraceptive efficacy, and long-term effects are still needed. © 2013 The Wildlife Society.