The purpose of this research is to explore the dynamics of cortisol regulation in the context of center-based child care by examining the impact of social context (large classroom vs. small group) and relationship quality with caregivers (conflict with mothers and teachers). We extend the research on children's physiologic stress system functioning in center-based child care by focusing on morning cortisol levels among young children living in poverty. While in high-quality center-based child care, children's cortisol levels decreased over the course of the morning—a result that contrasts with findings in previous research with middle-class children, for whom cortisol typically increased over the course of the day while attending center-based child care. Cortisol levels were further reduced when children were moved from a large classroom environment to a small group context. Relationship conflict with mothers and teachers moderated these effects. Children who had high conflict with their mother exhibited cortisol levels that remained higher (decreased less) over the course of the morning, and children who had high conflict with their teacher exhibited cortisol levels that remained higher (decreased less) in response to the small group context. These results indicate that high-quality child care has the potential to support reduced stress among children living in poverty, at least as indicated by adrenocortical activity.