Improving captive animal welfare and maintaining its behavioral competence for future conservation purposes is of highest priority for zoos. The behavior of an aggressive male drill (Mandrillus leucophaeus poensis) was assessed in Barcelona zoo. The two-year study presented in this article examines the effects of introducing changes in the exhibit of the drill in order to improve its welfare by analyzing scan behaviors. First, a partial visual barrier was applied which proved to be insufficient to decrease the long-term stress indicators assessed. After this a feeding enrichment program was implemented. The results obtained supported our hypothesis that feeding and explorative activities would increase whereas apathetic and stereotypic behavior would decrease. However, the visitor-directed aggression did not vary, indicating that more profound structural modifications were needed to reduce the negative impact of the agonistic interactions between the drill and the public. The study emphasizes the usefulness of environmental enrichment evaluations in assessing captive animal welfare.