The relation among children's evening activities, behavioral characteristics, and activity of the hypothalamic-pituitary-adrenocortical axis was assessed in normally developing children ages 7 to 10 years. Salivary cortisol at bedtime was compared on evenings when children had structured activities outside of the home with unstructured evenings at home in relation to parental reports of children's behavioral characteristics. Participating in evening activities, particularly sport activities, was associated with small increases in bedtime cortisol levels in boys but not in girls. Differences in cortisol on activity versus no-activity nights were negatively related to children's social isolation. These results show that in studies with children, nights on which participants engage in sport activities should be avoided when collecting ambulatory measures of salivary cortisol concentrations.