Dominant male adult vervet monkeys have whole-blood serotonin concentrations approximately twice those of subordinate adult males. We examined the effects of spontaneous and induced changes in social status, temporary isolation from the social group, and membership in single male groups on whole-blood serotonin concentrations. We found that in male vervet monkeys, elevated blood serotonin concentration is a state-dependent consequence of active occupation of the dominant male social position, and we believe that a reinterpretation of the significance of hyperserotonemia in humans may be warranted.