To reduce male–male aggression in collared lemurs (Eulemur collaris) at Disney's Animal Kingdom®, a clinical trial of leuprolide acetate (Depo-Lupron®) injections through one breeding season (24 wk), followed by surgical castration after the breeding season was implemented. Daily behavioral observations to record occurrences of aggression were conducted before and during the injection period, as well as after castration. Fecal sample collection began 2 wk before the first injection and continued throughout the clinical trial to determine testosterone metabolite concentration. Samples were collected 3 times per week. Fecal collection and behavioral observation continued for 5 mo after castration. Fecal samples from 3 intact males from other institutions were obtained to compare testosterone values. While the lemurs were treated with Depo-Lupron, testosterone concentrations spiked immediately after injection, and then returned to pretreatment, nonbreeding-season levels after varying lengths of time. Analysis of the behavioral data indicated that aggression was not significantly reduced with Depo-Lupron treatment. However, after castration, contact aggression and wounding decreased to zero. Although the sample is small, this study presents the first documented data on the effects that chemical treatment with Depo-Lupron and surgical castration have upon hormone levels and aggressive behavior in eulemurs.