Plasma cortisol was determined on 40 full-term infants on the third day of life. The relationships between cortisol and sex, birthweight, Apgar score, circumcision, and behavioral state were examined. Plasma cortisol was found to correlate most closely with behavioral state. In three infants plasma cortisol was assessed at weekly or fortnightly intervals from one to twelve weeks of age. Cortisol levels were found to vary more closely with behavioral state at the time the blood sample was taken than with changes in chronic irritability.