Physical inactivity has been identified as a risk factor for coronary heart disease (CHD) and Obesity (Pate, R. R. et al., 2002), with adolescents particularly at risk. The ‘Take PART’ study examines the relationship between physical activity, the transtheoretical model (TTM) of behaviour change and social support for physical activity, while also providing physical activity and health behaviour data. A self-report questionnaire was used to quantify physical activity level, the TTM variables and social support. Regular physical activity was defined as > 60 minutes of moderate-vigorous physical activity (MVP A) on four or more days per week. Physical measures included body mass index and aerobic fitness (20-MST). Structural Equation Modeling was used to test a hypothesised relationship between the TTM and social support. A total of 939 students (age 16.04 yrs ± 0.66] mean ± s), 60% female, participated in this cross sectional study. Sixty five per cent of participants were not regularly active (NRA), with more NRA females (70% vs. 58%,/? <0.001). The NRA group exhibited lower aerobic fitness (46.5 ± 24.3 runs Vs 62.7 ± 29.9 runs,/) <0.001) and participated in less leisure time physical activity (260 Vs. 483 minutes per week, /><0.001). Five percent of the adolescents were classified as obese, with 17% overweight. The stage distribution of the overall sample was Precontemplation (3.1%); Contemplation (11.8%); Preparation (34%); Action (9.1%); Maintenance (41 .9%). A hypothesised TTM model, with the new model replacing pros and cons with social support, explained 70% of the variance in physical activity ('/2(15,N=675)=18.71,/> =0.23, GFI=0.99, AGFI=0.98, RMSEA=0.02). Regular physical activity was associated with higher aerobic fitness, leisure time physical activity, self-efficacy and social support. Interventions need to focus on increasing levels of physical activity as a weight management strategy for youth, whilst incorporating the determinants of self-efficacy and social support.