This research examines (a) the interrelationships between body mass index (BMI), the direct measures of the Theory of Planned Behaviour (TPB) and physical activity intention and (b) the potential mediation effects of the direct measures of the TPB in the relationship between BMI and physical activity intention in a sample of Australian adults. A total sample of 1,062 respondents participated in a computer-assisted telephone-interview (CATI) survey comprised of a standardised introduction; questions regarding TPB and physical activity; and standard demographic questions. BMI for each participant was calculated from self-reported height and weight. Separate regression analyses were performed to examine the mediating effects of each of the direct measures of the TPB on the predictive relationship between the BMI and physical activity intention, as proposed by Baron and Kenny (Journal of Personality and Social Psychology, 51(6), 1173 - 1182, 1986). Findings indicated that the direct measure of attitude and perceived behavioural control mediated the relationship between BMI and physical activity intention. However, the direct measure of subjective norm failed to act as a mediating mechanism. To date there has been no research that has examined the mechanism by which body mass may affect physical activity behaviour. Given the current focus for health promotion specialists on promoting physical activity as a strategy for reducing overweight and obesity, a theoretical understanding of weight-related barriers to physical activity may aid in the development of future interventions and community physical activity programs, particularly those targeting overweight and obese populations.