The dominance hierarchy has repercussions for a monkey's sexual behaviour and endocrine state. Here we report on neural mechanisms that are sensitive to a monkey's status in the social hierarchy, and which may regulate not only its endocrine function but its sexual responsiveness to its own hormones. During the initial phase of group formation, 5-hydroxyindole acetic acid, the metabolite of serotonin, increases in the cerebrospinal fluid of monkeys which become subordinate (all groups), but decreases in monkeys which become dominant (two out of three groups) and shows no changes in intermediate-ranking animals (five out of seven). Homovanillic acid, a metabolite of dopamine, may also increase in the cerebrospinal fluid of monkeys that become subordinate (two out three groups). In the initial period of group formation these changes in transmitter metabolites do not parallel changes in cortisol. However, in the established social groups, both 5-hydroxyindole acetic acid and plasma cortisol are related to the social hierarchy, being greater in those monkeys that are subordinate, but homovanillic acid shows no consistent change. Although subordinate monkeys receive more aggression than others in their group, fluctuations in 5-hydroxyindole acetic acid do not correlate with aggressive behaviour, and are equally high on days when no aggression occurs. Dominant males, however, had higher 5-hydroxyindole acetic acid levels on days when they were involved in agonistic encounters. In the established social hierarchy therefore, elevated levels of the serotonin metabolite in cerebrospinal fluid seem reflect a "state"-dependent consequence of occupying a position of low social status.