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 FY01 (a) characterized a polymorphism in the MAO-A receptor gene for rhesus monkeys, with 3 distinctive alleles documented among members of the LCE rhesus monkey colony; (b) identified rearing condition differences in the development of circadian rhythms in activity for rhesus monkeys over the first 5 months of life, as well as differences in acquisition and performance on a variety of cognitive and motivational tasks during the first 18 months of life (c) using MRI scans, characterized rearing condition differences in total brain volume and in specific neocortical and limbic brain regions in yearling rhesus monkeys; (d) documented rearing condition differences in relative social dominance among juvenile rhesus monkeys that changed in their adolescent and adult years; (e) demonstrated a predictive relationship between measures of orientation and attention at 2 weeks of age and amount of alcohol consumed in early adulthood by rhesus monkeys independent of sex or type of early experience; (f) documented a strong positive relationship between brainstem serotonin transporter availability and alcohol consumption in young adult rhesus monkeys; (g) demonstrated a sensitization of CRH response over repeated social separations in juvenile rhesus monkeys; (h) identified a predictive relationship between CSF 5-HIAA concentrations of free-ranging rhesus monkey males at 3 years of age and levels of aggression, reproductive success, and mortality up to 10 years of age; and (j) documented predictive relationships between individual differences in measures of capuchin monkey infant behavioral activity and HPA reactivity at 6 and 12 months of age, respectively, which in turn were predictive of individual differences in a variety of specific measures of social behavior and environmental exploration over the next 5 years of life that parallelled previous findings in rhesus monkeys, despite major differences between the two species in overall rates of social activity and in social group structure and dynamics.