One pathway by which infant mammals gain information about their environment is through ingestion of milk. We assessed the relationship between stress-induced cortisol concentrations in milk, maternal and offspring plasma, and offspring temperament in rhesus monkeys. Milk was collected from mothers after a brief separation from their infants at 3–4 months postpartum, and blood was drawn at this time for both mothers and infants. Offspring temperament was measured at the end of a 25-hr assessment. Cortisol concentrations in milk were in a range comparable to those found in saliva, and were positively correlated with maternal plasma levels. Mothers of males had higher cortisol concentrations in milk than did mothers of females, and cortisol concentrations in maternal milk were related to a Confident temperament factor in sons, but not daughters. This study provides the first evidence that naturally occurring variation in endogenous glucocorticoid concentrations in milk are associated with infant temperament. © 2010 Wiley Periodicals, Inc. Dev Psychobiol 53: 96–104, 2011.