|Abstract||A large percentage of the US population is physically inactive, increasing the risk of cardiovascular disease. Mood states associated with exercise are likely to influence future exercise adoption and adherence and merit further investigation. The present study examined how an individual's physical activity level and the intensity of acute bouts of exercise affect mood pre- to post-exercise and whether these changes correspond to beta-endorphin levels. Participants were 12 college students (4 male, 8 female) divided into active and inactive groups. Each participant completed a 20-minute bout of treadmill exercise at low intensity (10% VO2max below VT) and high intensity (5% VO2max above VT). Before and after each bout, subjects completed the Subjective Exercise Experience Scale. A blood draw was also done post-exercise to determine plasma beta-endorphin levels. No difference in affect response was observed between groups. Low intensity exercise elicited a significant increase in positive well-being, as well as smaller decreases in fatigue and psychological distress. Beta-endorphin levels were higher following the high intensity bout compared to the low intensity bout. No significant relationship was observed between beta-endorphin levels and changes in mood. The findings of this study suggest that low intensity exercise is sufficient for improvements in mood states regardless of fitness level, and that these improvements are not correlated with beta-endorphin levels. Encouraging individuals to begin a low intensity exercise program could potentially decrease the prevalence of physical inactivity due to the associated mood improvements.