This study examined the increase in salivary cortisol from midmorning to midafternoon in 151 children (3.0-4.5 years) in full-time home-based day care. Compared to cortisol levels at home, increases were noted in the majority of children (63%) at day care, with 40% classified as a stress response. Observations at day care revealed that intrusive, overcontrolling care was associated with the cortisol rise. For girls, the cortisol rise was associated with anxious, vigilant behavior, while for boys the rise was associated with angry, aggressive behavior. Child behavior did not mediate or moderate relations between care quality and the cortisol rise, except for evidence that boys scoring low on angry, aggressive behavior were more sensitive to variations in warm, supportive care than boys scoring high on this behavior.