OBJECTIVES: This study aimed to examine how general motives from self-determination theory (SDT; Deci & Ryan, 1985) influence intentions to engage in physical activity within the Theory of Planned Behaviour (TPB; Ajzen, 1985). It was hypothesized that the general motives will influence intentions only when mediated by the specific cognitions of attitude, subjective norms and perceived behavioural control (PBC) from the TPB. DESIGN: A cross-sectional study assessing psychological variables from two theoretical perspectives. METHOD: Self-report questionnaires were administered to 1088 children aged 12-14 years. The children's intentions, attitudes, subjective norms and PBC towards participating in physical activity were assessed using a TPB questionnaire. A modified verson of Ryan and Connell's (1989) perceived locus of causality (PLOC) inventory was used to measure controlling and autonomous motives for participating in physical activity. RESULTS: These data were analysed using structural equation modelling. The resulting well-fitting model demonstrated that attitude and PBC mediated the influence of autonomous motives to perform physical activity on physical activity intentions. The presence of autonomous motives resulted in the effects of the controlling motives being attenuated to zero. CONCLUSIONS: The present results indicate that general autonomous motives to participate in physical activity act as sources of information when childen make their judgments regarding their specific attitudes and PBC. Attitudes and PBC are necessary to translate these general motives from SDT into intentions in the TPB. In terms of targets for intervention, practitioners may positively influence intentions by providing a choice of physical activities to foster increased autonomy in children.