This project addresses the current epidemic in obesity and physical inactivity among adolescents in the United States. The goal of the research is to develop effective ways of encouraging adolescents to become and remain physically active. In particular, this project is concerned with identifying adolescents who are reluctant to exercise because they have a high sensitivity to unpleasant feelings while exercising at higher intensities. Once identified, these adolescents can be targeted with an intervention that is designed to teach them to exercise at anintensity level that will generate pleasant feelings and therefore make it more likely that they will seek out opportunities to be physically active. The specific aims of this study are to: 1) evaluate the impact of a novel intervention delivered via school-based physical education (PE) on adolescents who have a high sensitivity to exercise-induced negative affect; 2) determine whether adolescents' tendency to feel uncomfortable during exercise is a stable trait that persists even in the face of an intervention; and 3) compare and contrast three alternative methods of measuring adolescents'sensitivity to exercise-induced affect. Healthy middle-school students who do not participate in team or individual competitive sports will be recruited and assessed to determine their existing predisposition toward exercise (i.e., reluctant exercisers and latent exercisers). The assessment will be conducted using three methods that have been used to measure individuals' propensity to experience positive affect in the face of a stimulus: 1) a pencil-and-paper assessment that measures tendency to respond to a challenge with positive affect; 2) electroencephalogram (EEG) to ascertain frontal cortical asymmetry; and 3) empirically assessed affective response to a standardized exercise task. Reluctant and latent exercisers will be assigned in equal numbers to one of two conditions. One condition will implement a PE-based intervention that differs from the traditional approach in that students will be instructed to exercise at an intensity that has been determined to elicit positive affect in that individual (based on baseline testing). In the other condition, students will be instructed to exercise at an intensity derived from standard formulas typically used in exercise prescriptions. It is hypothesized that the non- traditional approach will increase reluctant exercisers'enjoyment of PE and also their level of participation in physical activity outside of PE. The latter will be determined using portable monitors (accelerometers) worn at baseline, after the intervention, and again 1 year after the end of the intervention. This study is relevant to the prevention of type 2 diabetes in that it addresses the mechanisms of physical activity behavior change among adolescents. This developmental period is typically characterized by declining participation in physical activity, and thus represents a critical period for intervention. The results will increase understanding about why some adolescents remain active while others do not and will test a novel intervention that may be more effective among reluctant exercisers.