In humans, CSF monoamine metabolite concentrations have been shown to vary as a complex function of age, sex, psychiatric diagnosis, and stress. To test for such relationships in rhesus monkeys, 28 subjects, reared either in anxiety producing peer-only groups or in mother-infant dyads, were studied at 6, 18 or 50 months of age. Each monkey underwent a series of four 4-day social separations, each followed by 3 days of reunion. Prior to and during the first and fourth separations, CSF was obtained from the cisterna magna and assayed for the serotonin metabolite 5-HIAA, the dopamine metabolite HVA, and the norepinephrine metabolite MHPG. CSF 5-HIAA showed an age-related decline which was greater in the mother-reared subjects. Peer-only-reared males had an increased 5-HIAA concentration relative to females, and higher 5-HIAA levels than mother-reared males. MHPG was also higher in peer-only-reared monkeys than in mother-reared subjects at all ages. In both groups HVA declined across the three ages, and MHPG increased from the 18- to the 50-month measurements. Both MHPG and 5-HIAA concentrations increased during the initial social separation, although only MHPG remained elevated across the repeated separations; HVA, on the other hand declined during social separation. These results are discussed in terms of established anxiety and aggression differences between peer-only and mother-reared monkeys.