This project investigates primate biobehavioral development through comparative longitudinal studies of rhesus and capuchin monkeys, with special emphasis on characterizing individual patterns of differential behavioral and physiological responses to environmental novelty and challenge, and on determining long-term developmental consequences for individuals of different genetic backgrounds reared in different physical and social environments. During the past year, prospective longitudinal studies of rhesus monkey infants differing in genetic pedigree but reared under identical nursery conditions revealed significant strain differences in blood CBC and serum chemistry, as well as differences in central serotonin metabolism and behavioral response to environmental challenge throughout the first 6 months of life. Other studies with rhesus monkey infants revealed significant effects of early social environmental factors on biobehavioral development in terms of alterations in neuroendocrine circadian rhythms and glucocorticoid sensitivity to routine caretaking activities to a far greater degree than previously presumed. Monkey infants reared under different social caretaking conditions also differed systematically in central serotonin metabolism from their first month onward; these differences were developmentally stable over the first 6 months. Field studies of rhesus monkey juvenile males showed that individuals with unusually low CSF levels of 5-HIAA, the primary central serotonin metabolite, exhibited unusually high levels of impulsive behavior, including unprovoked and escalating aggression. Most of these impulsive males were physically expelled from their natal troop well before puberty, with a very high subsequent mortality rate. Immunological studies of captive juvenile and adolescent rhesus monkeys demonstrated that low CSF 5-HIAA was associated with suppressed immunoreactivity, while PET scans of adolescent males with high vs. low CSF 5-HIAA levels (and low vs. high levels of unprovoked aggression) revealed significant differences in measures of regional cerebral glucose metabolism. Longitudinal studies of biobehavioral development in capuchin monkeys disclosed significant predictive relationships between early activity state profiles and later behavioral proclivities.