Amid growing evidence that American youth are not sufficiently active to promote health and prevent disease, a call has been issued for innovative programs that will stimulate adolescents to engage in increased levels of exercise. Adolescent females, in particular, stand to benefit from successful interventions for a number of reasons; namely, that they tend to participate in lower levels of physical activity as compared to adolescent males, and they are at higher risk for several conditions associated with sedentary behavior (e.g., osteoporosis, obesity). One current focus in intervention research has been on testing the utility of identified correlates of exercise as mechanisms for change in exercise behavior. Specifically, a number of psychosocial factors have been put forth as possible mediators of exercise behavior, including perceived self-efficacy for exercise, social support for exercise, perceived benefits of and barriers to exercise, and enjoyment of exercise. The present study analyzes data from a pilot of a school-based intervention designed to promote physical activity among sedentary adolescent females. The study was designed to determine first whether the intervention brought about an increase in physical fitness and secondly whether the participants demonstrated the expected changes in the set of psychosocial variables described above. Twenty-two sedentary adolescent females were recruited from a local high school. Criteria for inclusion in the study included the following: 1) currently exercising fewer than 3 times per week; and 2) a score at or below the 70th percentile for age-adjusted cardiovascular fitness. Participants were primarily Caucasian (n=16), with 3 Hispanic and 3 Asian. At Baseline and Follow-up, study participants completed a questionnaire containing standardized scales to assess psychosocial variables and performed a ramp-type progressive cycle ergometer test to assess cardiovascular fitness. Following baseline testing, participants were randomly assigned to a specially designed physical education class (n=9) or to a wait-list control condition (n=13). Analysis of Variance revealed that the intervention had a significant effect on cardiovascular fitness (F=4.94, p < .03), with intervention participants achieving a greater increase in fitness. In contrast, none of the psychosocial variables were significantly impacted by the intervention. This study presents evidence that contradicts the popular assumption that interventions should strive to modify psychosocial influences on exercise as a means for bringing about increased cardiovascular fitness.Keyword(s): exercise/fitness, high school issues, wellness/disease prevention