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Population of wildlife, such as California ground squirrels, can grow to the extent that they come in conflict with humans. Contraception is a method of population management under investigation that may be useful in situations where neither leaving the animals uncontrolled nor lethal control are apropos. In this study, we tested the use of a single-injection gonadotropin releasing hormone (GnRH) immunocontraceptive vaccine in urban California ground squirrels. We monitored the effects of treatment for two breeding seasons. Immunization reduced the proportion of females lactating by 91% the first year and 96% in the second year. Testicular development was inhibited 35% the first year and 89% the second year. There is a delay of several months from the time of injection to inhibition of testes development. Reduction in the number of juveniles born per adult as determined by a visual count index was 9% the first year and 66% the second year. This study shows that the single-shot GnRH vaccine is over 90% effective for at least 1.5 years and requires several months after immunization for contraceptive effect. Because the immuni7ationrequires injection, it is labor intensive, but it is much more practical than treatments requiring multiple administrations to the same animal. GnRH immunocontraception may be a useful tool in rodent population management in certain circumstances.