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 FY96 (a) replicated and extended previous findings regarding early experience effects on the development of circadian rhythmicity in glucocorticoid release in rhesus monkey infants; (b) demonstrated the efficacy of surrogate-peer rearing procedures for the development of species-normative patterns of behavior and physiology in nursery-reared rhesus monkey infants and juveniles; (c) successfully cross-fostered rhesus monkey infants selectively bred for extreme (either high or low) CSF levels of 5-HIAA (the primary central serotonin metabolite) to foster mothers of differing behavioral and physiological propensities for long-term longitudinal study; (d) demonstrated a significant relationship between serotonin metabolism rates and sleep-wakefulness patterns in group-living rhesus monkey juveniles and adults; (e) documented systematic seasonal variation in serotonin metabolism in free-ranging rhesus monkey adolescent and adult males; (f) characterized changes in heartrate and heartrate variability associated with reconciliation behavior following aggressive exchanges in rhesus monkey adult females; and (g) characterized patterns of adrenocortical response to brief separation in infant and juvenile capuchin monkeys and demonstrated a predictive relationship between magnitude of cortisol response and subsequent propensity for object manipulation and proto-tool use behavior in this primate species.