The present study examined the cognitive processes by which perceived locus of causality influences participation in leisure time physical activity. Based on the theoretical tenets of self-determination theory, it was hypothesized that deliberative modes of information processing and past behaviour will account for the direct effects of perceived locus of causality on effort and physical activity. Data from self-report questionnaires were analysed using confirmatory factor analysis and path analysis. Results demonstrated that perceived locus of causality influences effort and physical activity participation via attitudes and perceived behavioural control. It was concluded that the addition of perceived locus of causality to the theory of planned behaviour increased the utility of the theory in predicting adherence to physical activity.