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 extensive neonatal testing of rhesus monkey infants reared either by their biological mothers or with peers revealed heritable influences on activity state profiles independent of rearing condition and rearing influences on CSF concentrations of the serotonin metabolite 5-HIAA independent of pedigree within the infants' first month of life; these differences were, in turn, predictive of differential behavioral, adrenocortical, and monoamine reactivity to environmental challenge at 6 months and beyond. An interaction between biobehavioral reactivity and chronic level of environmental stress was found to be predictive of injury rates among members of a free-ranging rhesus monkey troop: high-reactive monkeys had lower injury rates than low-reactive monkeys under low stress conditions, but significantly higher injury rates than their low-reactive counterparts under high stress conditions. Thus, high reactivity appears to be a protective factor in the absence of chronic stress but paradoxically becomes a significant risk factor for injury under conditions of high environmental stress. A comparison of heartrate patterns among wild-living adult female rhesus monkeys revealed strong familial influences, with closely related females showing significantly greater concordance of heartrate parameters than distantly related females, who in turn showed greater concordance than nonkin. A separate study of captive group-living female rhesus monkeys demonstrated that low CSF levels of 5-HIAA were associated with low social dominance, poor social competence, and a high incidence of impulsive aggression, replicating associations previously reported for both captive and wild- living rhesus monkeys males. Finally, major methodological advances were achieved in biological sampling techniques, telemetric monitoring of physiological functioning in free-ranging monkeys, and primate PET-scan brain imaging.