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. Research carried out during FY97 (a) identified 3 distinctive clusters of prototypical behavioral response to novelty in month-old rhesus monkey infants; (b) demonstrated accelerated development of orienting and motor capabilities in nursery-reared rhesus monkey infants who received supplements of essential fatty acids to their standard formula diets beginning at birth; (c) characterized significant differences in behavioral development among rhesus monkey infants reared by mothers who differed systematically in chronic CSF concentrations of 5-HIAA, the primary central serotonin metabolite; (d) documented significant deficits in courtship and reproductive behavior in free-ranging rhesus monkey adolescent and young adult males with unusually low chronic CSF 5-HIAA concentrations; (e) described systematic associations between the relative incidence of different types of aggressive behavior and CSF levels of 5-HIAA and testosterone in free-ranging rhesus monkey adult males; (f) characterized short- and long-term predictive relationships between different early behavioral and activity state profiles and subsequent biobehavioral developmental trajectories in capuchin monkey infants and juveniles; and (g) demonstrated a predictive relationship between individual differences in prototypic biobehavioral response to challenge among capuchin monkey infants and their subsequent proclivity to engage in exploratory and proto-tool use behaviors.