The purpose of this study was to assess sex differences in stress responses to cooperative and competitive activities within same-sex dyads. Participants included 90 undergraduate college students. Salivary cortisol (CORT) was collected as a measure of stress reactivity in the hypothalamic-pituitary-adrenal (HPA) axis. Salivary alpha-amylase (sAA) was collected as a measure of stress reactivity in the sympathetic nervous system. Data were analyzed using a 2 X 2 analysis of variance, with condition (cooperative activity, competitive activity) and sex (male, female) as independent variables and salivary cortisol (CORT) and alpha-amylase (sAA) as the dependent variables. Results indicated a significant interaction between sex and condition for change in CORT from time 1 (baseline measurement) to time 2 (measurement immediately following shared activity). There was a significant difference in rate of CORT change from time 1 to time 2 for women in cooperative condition compared to the competitive condition. Specifically, women engaging in cooperative activity showed greater decreases in CORT from time 1 to time 2 than women engaging in competitive activity. There was no significant change in CORT for men during cooperative or competitive activity. No significant effects of condition or sex on sAA were observed.