Associations between behavioral inhibition and activity of the hypothalamic-pituitary-adrenocortical (HPA) system, a stress-sensitive neuroendocrine system indexed by salivary cortisol, have varied widely across studies. In the current study, we examined the role of peer social experiences in moderating patterns of association between inhibition/risk-aversion and cortisol reactivity. As expected based on previous research, preschool children (N = 165, 78 boys, 87 girls, 3.0-5.0 years) had significantly different social experiences in their preschool classrooms depending on temperament. Highly inhibited/risk-averse children were less socially integrated, less dominant, and less involved in aggressive encounters than both average and highly exuberant/risk-seeking children, but they were no more likely to be peer rejected. Highly exuberant children were more dominant, exhibited anger more often, and had friendships characterized by higher conflict. Cortisol levels fell from fall to spring for average and highly exuberant children but not for highly inhibited children. Unexpectedly, for highly inhibited children, having friends and being more dominant and popular than other highly inhibited children was associated with increasing cortisol levels over the school year. In contrast, highly exuberant children who were less socially integrated than other highly exuberant children maintained higher cortisol levels. Results indicate that the types of social experiences that affect stress-responsive biological systems may differ markedly for highly inhibited and highly exuberant children. (Contains 2 tables and 3 figures.)