To identify the degree to which the social cognitive theory constructs targeted by the Planning to Be Active Program (PBA) were mediators of moderate-intensity exercise. High school students in treatment and comparison groups received activity-based physical education. The treatment group also received PBA, which develops self-regulation skills to promote leisure-time exercise. PBA increased self-regulation scores, social situation scores, and moderate-intensity exercise. Self-regulation and social situation mediated moderate-intensity exercise at posttest. As mediators, self-regulation and social situation help explain how PBA produces increases in moderate-intensity exercise.