The effects of stress on exercise behavior in community-residing women exercising on their own were assessed. Participants (N = 82) completed a background questionnaire and kept exercise diaries and Weekly Stress Inventories (P. J. Brantley, G. N. Jones, E. Boudreax, & S. L. Catz, 1997) for 8 consecutive weeks. During weeks with a high frequency of stressful events, participants exercised for less time and reported lower self-efficacy for meeting upcoming exercise goals. During weeks of high perceived stress, participants exercised significantly fewer days, omitted more planned exercise sessions, were less satisfied with their exercise, and had lower self-efficacy for meeting exercise goals. Findings suggest that perceptions of stressful events and cognitive reactions to missed exercise may play a significant role in mediating exercise behavior and support the view of exercise relapse as an ongoing process.