AbstractThe simultaneous effects of naturally occurring individual differences in maternal care and maternal peripartum stress on infant development have been sparsely reported in nonhuman primates. In this work, we used a comparative approach to assess how changes in peripartum maternal excreted cortisol levels and the quality of mother–infant interactions correlate with infant behavioral development in group‐living rhesus and Japanese macaques. We tested the hypothesis that peripartum maternal stress was associated with infant behavioral characteristics during development. Due to the difference in mothering style between the two species, we provided separated analyses for two groups. A sample of mother‐infant pairs (Japanese macaques,N = 14; rhesus macaques,N = 10) was observed during the first 3 months of the infant's life. Follow‐up observations (at 5, 7, and 9 months of age) were collected for the infants. Maternal cortisol levels were measured during the peripartum period. We found preliminary evidence that maternal peripartum stress and differences in key components of maternal behavior are associated with infant behavior throughout the developmental phase. We also provided a working hypothesis regarding maternal behavior and maternal stress as factors playing unique roles in different components of infant behavioral development. © 2005 Wiley Periodicals, Inc. Dev Psychobiol 48: 1–9, 2006.