Mice maintained on a basal casein diet supplemented with 4% L-tyrosine potentiated L-DOPA effects on aggression. At low doses (12.5-25 mg/kg) L-DOPA increased aggression whereas at high doses (50-100 mg/kg) it decreased aggression. 5-HTP (50-200 mg/kg) produced a dose-dependent decrease in aggression and motor activity which was antagonized by pretreatment with dietary L-tyrosine (4%) or L-DOPA (50 mg/kg). L-DOPA induced reductions in motor activity were, in turn, antagonized by 5-HTP. Increases in motor activity following d-amphetamine (3 mg/kg) were sharply reduced by 5-HTP (50-100 mg/kg), but 5-HTP potentiated reductions in aggression following d-amphetamine. The concentration in brain of tyrosine, DOPA, dopamine (DA), noradrenaline (NA), DOPAC, HVA, tryptophan, serotonin (5-HT), and 5-HIAA were obtained following drug and diet treatments. The changes observed, particularly in DA and 5-HT metabolites, provide further evidence for an inhibitory role of brain 5-HT systems in the mediation of the behavioral effects of d-amphetamine and the catecholamine precursors, L-tyrosine and L-DOPA.