Cortisol is a glucocorticoid hormone with a secretory baseline pattern that is circadian in nature. It is additionally released in response to physiological or psychological stressors as the end product of the hypothalamic-pituitary-adrenocortical (HPA) axis. Increases in cortisol following vigorous exercise are believed to be beneficial for metabolic processes and support protein synthesis. It has recently been documented that adult males and females have distinct cortisol responses to exercise (Putnam, Chrousos, Nieman, & Rubinow, 2005). While both exhibited increases in cortisol levels following an acute bout of medium to high intensity exercise, the response for males was significantly greater than that for females. Males also appeared to have a more rapid response than females. The purpose of this study was to investigate cortisol levels in a population of young boys and girls (M = 34 months, SD = 6 months) following a 30 min physical play session. The comparison with adult male and female exercise-induced cortisol levels would highlight possible developmental trends in the HPA axis response to physical activity. Saliva samples were collected at 9:45 a.m. and 10:35 a.m., pre- and post-physical play, frozen, and then sent to a laboratory to be assayed. A mastery motivational climate was used to set-up the playground with various motor skill activities which allowed the children to be autonomous in their play. The investigators also modeled engagement in physical activity during the play session. A repeated measures ANOVA was conducted using the cortisol levels measured in the saliva samples collected. The results showed no differences between boys (n = 10) and girls (n = 12) at this age (p = .93), and no increase in cortisol following the physical play session (p = .55). Since the mastery motivational climate utilized significantly increased heart rate (HR) and time spent in vigorous activity, it can be concluded that lack of engagement was not a factor in this result. Further investigation may reveal the age at which cortisol responses to exercise transition to more mature gender-specific patterns.Keyword(s): early childhood, exercise/fitness/physical activity, gender issues