The systemic administration of L-DOPA and carbidopa to six rhesus and four squirrel monkeys produced an initial period of depressed activity followed by increased locomotion, hypervigilance, involuntary oral-facial movements and a gnawing syndrome. The squirrel monkey exhibited a depressed phase, locomotor stimulation, searching behavior, stereotypic grooming and gnawing syndrome. Most of these activities were prevented by pretreatment with 0.1 mg/kg haloperidol. Bilateral injection of 100 microgram dopamine into the mesolimbic forebrain of four squirrel monkeys also produced an initial depression followed by hyperactivity similar to that produced by L-DOPA, but without gnawing. A stereotyped submissive or juvenile posturing occurred in three animals. These DA-induced activities were blocked by 0.1 mg/kg haloperidol. Similar injection of 100 microgram L-norepinephrine produced a profound depression followed by moderate activity coupled with loss of extensor muscle strength in the legs. Bilateral injection of 300 microgram dopamine into the nucleus accumbens of a rhesus monkey produced stereotypic pacing. These data confirm in primates the importance of dopaminergic mechanisms of the mesolimbic forebrain in locomotor activity and behavior.