Purpose: This study tested the health action process approach (HAPA) as a motivational model for physical activity in a longitudinal sample of 483 adults (275 males, 208 females) over a 10 week period. Methods: Participants' action self-efficacy, decision balance for exercise, perceived risk awareness and intention were measured at baseline (Time 1). Maintenance self-efficacy, action planning and action control were measured 5 weeks later (Time 2). Physical activity behavior was measured 10 weeks after baseline (Time 3). Structural equation modeling was used to investigate the HAPA model. Results: The first model showed that intention was a significant predictor of physical activity behavior. The second model, which included the volitional components of the HAPA model, did significantly increased the proportion of behavior explained. The third model showed that intention, maintenance self-efficacy and action control were positive predictors of physical activity behavior. Conclusion: Findings suggest that HAPA provides a useful framework for identifying determinants of physical activity intention and behavior within a group of middle-aged adults.