The goal of this dissertation research was to describe the relationships among psychosocial variables, physical activity and physical fitness, and biological measures indicative of cardiovascular health in Mexican American male adolescents using a biobehavioral model. One aim of the research was to describe the predictive relationship of psychosocial variables, perceived benefits, perceived barriers, self-efficacy, and interpersonal influences, on physical activity and physical fitness. A second aim described the predictive relationship among physical activity and physical fitness and the amount of body fat and levels of biological markers indicative of endothelial function in this population. Study participants were 28 Mexican American male adolescents ages 15-19. Psychosocial variables were assessed using instruments developed for adolescents by Pender. Physical activity was measured by a 3-day accelerometer recording of activity counts while physical fitness was measured with cycle ergometry withVO2 max. Biologic measures indicative of cardiovascular health included serum leptin, CRP, adiponectin. Fat mass was assessed using BMI and DEXA scans. Findings demonstrated partial support for the model. Psychosocial variables predictive of physical fitness included perceived benefits of action and interpersonal influences. Perceived benefits of exercise significantly predicted physical fitness, explaining 50% of the variance in physical fitness scores while exercise norms, a measure of interpersonal influence, predicted 17% of the variance. Self-efficacy did not meet criteria as a mediating variable; it directly predicted physical activity. Physical activity predicted 15% of the variance in body fat measured as BMI percentile. Physical fitness predicted Leptin levels accounting for 23% of the variance. Physical fitness also predicted 51% of the variance related to the DEXA-derived body fat measurement and 18% of the variance related to BMI. Additional trends were identified including lack of parental support for exercise. Although the study participants reported high acculturated levels, language spoken at home indicated that the family was less acculturated which may have accounted for the lack of parental support. Higher acculturation levels were also significantly associated with increased perceived benefits of action and higher BMI levels. In conclusion, this study suggests that selected psychosocial variables including interpersonal influences should be considered in designing research with Mexican American adolescent males. In addition results suggest that objectively obtained measures of physical fitness and activity are in part predictive of measures of endothelial function and body fat.