At times severe, and occasionally fatal, aggression plays an intrinsic role in chimpanzee behavior and social dynamics, particularly among male chimpanzees in both managed and free-ranging troops. At the Los Angeles Zoo, one adult male's natural aggressive behavior developed into unmanageable violence during a period of social and emotional instability consequent to the lack of an established alpha male in the colony. The severity and duration of resulting attacks on a subdominant member of the community, despite environmental and behavioral modification, indicated the need for psychopharmaceutical intervention. Prior treatment of this animal with haloperidol and gabapentin had produced undesirable side effects. Administration of citalopram hydrobromide, a selective serotonin reuptake inhibitor, successfully reduced both the intensity and duration of this male chimpanzee's attacks upon a conspecific animal with minimal observable side effects or adverse behavioral changes.