The effects of dominance rank on the behavioral responses to drugs that enhance central serotonergic function were examined in 45 adult male vervet monkeys living in 15 stable social groups. Each group contained 3 adult males, 3 adult females, and their immature offspring. Dominance rank was assessed by measuring success in intermale agonistic encounters. In every group one male was clearly the dominant, or alpha male, and the other two males were subordinate. Males from 5 groups received 3 doses of the serotonin reuptake inhibitor fluoxetine (0.5, 1.0 and 2.0 mg/kg/day); those from a second set of 5 groups received 3 doses of the receptor agonist quipazine (0.25, 0.50 and 1.0 mg/kg/day); those from a third set of 5 groups received the serotonin precursor tryptophan (10, 20 and 40 mg/kg/day). The 3 drug treatments produced strikingly similar behavioral effects. Each produced dose-dependent increases in approaching, grooming, resting and eating and decreases in locomoting, avoiding, being vigilant and being solitary. Dominant males were significantly more responsive behaviorally to all 3 drugs than were subordinate males: the increase or decrease in each behavioral measure was larger in dominant than in subordinate males. In combination with previous studies, these data suggest that dominant and subordinate males differ in the drug sensitivity of their serotonergic systems.