This study tested several relationships predicted by the Health Action Process Approach (HAPA) in a sample of 175 generally healthy, inactive, middle-aged women (40-65 yrs old) over a 12 week period. Participants' physical activity, risk perceptions, outcome expectancies, action self-efficacy and intention were measured at baseline. Planning and maintenance self-efficacy were measured 4 weeks later. Physical activity behaviour was measured 12 weeks after baseline. The HAPA relationships were examined using a structural equation model. The data fit the model well and revealed several significant relationships. Action self-efficacy was the best predictor of intention. Maintenance self-efficacy was the best predictor of planning and behaviour. Contrary to the tenets of HAPA and to past research, planning did not predict behaviour. Overall, HAPA provides a useful framework for identifying determinants of physical activity intentions and behaviour within a group of inactive, middle-aged women.