The behavioral effects of a hallucinogen, 5-methoxy N,N-dimethyltryptamine (5-MeODMT), were studied following acute and chronic administration of the substance to two selected members of a Stumptail macaque social colony of four adult monkeys. Intramuscular injection of 5-MeODMT in doses varying from 5-250 μg/kg demonstrated dose-dependent induction of abnormal behavior and attenuation of normal affiliative behavior. Abnormal behavior induced by 5-MeODMT included dog-like "wet shakes," involuntary limb jerks, stereotyped behavior, and hypervigilance. 5-MeODMT induced dose dependent decreases in social groom and initiated social activity while self grooming either increased or remained unchanged. Dose dependent increases in submissive gesture were also seen following administration of the hallucinogen. Chronic administration of 250 μg/ kg 5-MeODMT for 12 days induced similar abnormal behavior and alterations of normal behavior. No definite signs of tolerance to any of these behavioral changes could be detected throughout the 12-day treatment period. Similarities of 5-MeODMT-induced behavioral changes in monkeys to the effects of this and other hallucinogens in humans are discussed. It is suggested that primate social colonies may offer an excellent paradigm to study the behavioral effects of hallucinogens as well as other psychotomimetics.