GonaCon™ Immunocontraceptive Vaccine targets the reproductive hormone gonadotropin releasing hormone (GnRH) secreted by the hypothalamus of the brain. Antibodies produced in response to the vaccine inactivate endogenous GnRH, which in turn eliminates stimulation of the pituitary gland and gonads in males or females. The resulting “immunocastration” renders animals unable to produce reproductive steroids or gametes. In previous studies, we demonstrated that GonaCon™ was effective in impairing fertility for up to 3 years in female deer without apparent side effects. However, detailed post mortem evaluations were not done. To better understand the entire physiological response of deer to GonCon™ and to establish if contraindications were associated with its use, we undertook the present study. To evaluate toxicity and safety, 7 does were given the standard single injection GnRH-KLH vaccine dose (1000 µg) delivered IM in 1 ml using AdjuVac™ adjuvant and compared to 6 does given a single control saline IM injection and 6 does given 3 injections of GonaCon™ at 2-week intervals per dose. The study was conducted for 20 weeks. Does were blocked by weight and randomly assigned within blocks to treatment groups. Blood samples were drawn immediately prior to vaccination and at 5, 10, 15, and 20 weeks post immunization. Blood was assayed for LH, testosterone and progesterone, and anti-GnRH titers, as well as hematology and blood chemistry. At each sampling period, the general health of the doe was observed and the injection site was inspected for the formation of abscesses or other tissue reactions. At Week 20, all deer were euthanized and evaluated at necropsy by veterinary pathologists and samples of lymph nodes, reproductive organs, lung, liver, heart, kidney, spleen, and brain were taken for histology. Aside from granulomata formation at the injection site, there were no significant contraindications or toxic effects associated with GonaCon™.