Previous studies comparing plasma cortisol levels in mother-reared and nursery-reared rhesus monkey infants under baseline and stress conditions have reported conflicting findings. Differences in subject age, procedures, and specific rearing history may account for many of the discrepant findings. In the present study, mother-reared infants from large social groups, peer-only reared animals, and infants reared with surrogates and limited peer contact were studied in different test conditions across the first 6 months of life. Infants were sampled under three conditions: following a neonatal assessment at Days 14 and 30, immediately upon capture on Day 60, and after 30-min isolation periods on Days 90, 120, and 150. Mother-reared infants exhibited higher cortisol levels on Days 14 and 30 than did both types of nursery-reared infants. In addition, Day 60 basal values of mother-reared infants were higher than those of both peerreared and surrogate/peer-reared infants. However, on Days 90, 120, and 150, both mother-reared and peer-reared infants exhibited higher cortisol levels in response to separation and 30-min isolation than did the surrogate/peer-reared infants. These differences may reflect group-specific variations in physical environment, capture time, feeding regimen, or diurnal HPA axis rhythms.