Fertility control offers a potential alternative to traditional methods for regulating the growth of overabundant wild ungulate populations. However, current technology is limited due to practical treatment application, undesirable side-effects and economic considerations. A promising non-steroidal, non-immunological approach to contraception involves the use of a potent GnRH agonist. Two experiments were conducted to evaluate the effectiveness of a GnRH agonist (leuprolide) for controlling fertility in captive female wapiti and to assess physiological and behavioural side-effects of the treatment. In Expt 1, the optimum dose of agonist treatment was determined by measuring serum LH response of eight female wapiti to four formulations of leuprolide (0, 45, 90 and 180 mg) administered as a subcutaneous (s.c.) bioimplant. In Expt 2, the effects of leuprolide on wapiti pregnancy rates, duration of suppression of serum LH and progesterone secretion, and short-term behavioural and physiological side-effects were evaluated. All concentrations of leuprolide in Expt 1 were equally effective in reducing serum LH to non-detectable values throughout the 130 day trial. In Expt 2, leuprolide administered before the breeding season was 100% effective at preventing pregnancy in treated females. Serum LH and progesterone were reduced to baseline values by day 92 and remained at this concentration for 195-251 days after treatment, and returned to pretreatment concentrations in the following breeding season. Reproductive behaviour rates were similar for treated and untreated wapiti for all behaviour categories for both the breeding and post-breeding seasons. Haematology and blood chemistry parameters of treated and un-treated females were similar, and seasonal intake and body weight dynamics appeared normal. In conclusion, leuprolide is a safe, effective contraceptive agent and can potentially suppress fertility in female wapiti for one breeding season.