In day-care centers, toddlers are often confronted with situations that challenge the limits of their social skills and involve emotional arousal. We investigated whether the environmental attributes of day-care centers modulate the emotional reactions of children regularly attending such group settings. Results show those stress reactions--as measured by steady cortisol increases--varied significantly among the 8 groups participating in the study. Higher cortisol increases were related to environmental attributes such as large group size (n >15), important age differences among children in a group and available surface per child inferior to 5 m2.