Previous work has found that many young children show different patterns of production of the hormone cortisol, which is sensitive to stress and challenge, on days when they are at child care compared with days when they are at home. At home, preschool age children typically show a decreasing pattern of cortisol production across the day which is the expected diurnal rhythm for this hormone. At child care many children show a rising pattern of cortisol production across the day. Lower child care quality has been associated with larger child care cortisol increases from morning to afternoon. The current study examined salivary cortisol at mid-morning, mid-afternoon, and evening on child care and weekend days in children attending the highest quality child care centers. Child-level classroom quality assessments were obtained using the Modified-Observational Rating of the Caregiving Environment. The rising pattern across the child care day was replicated, although in a smaller proportion of the children than previously reported. Variation in the quality of the child's niche or microclimate predicted these cortisol increases. When children returned home from child care, cortisol levels returned to levels observed on non-child care days even for children who showed the rising cortisol pattern during child care. (Contains 3 tables and 2 figures.)