Primary care is a promising venue to build patient motivation and confidence to increase physical activity (PA). Physician PA counselling has demonstrated some success; however, maintenance of behaviour change appears to require more intensive interventions. In reality, most physicians do not have the necessary training nor the time for this type of counselling. The purpose of this paper is to outline the rationale, methods, and interventions for the ongoing physical activity counselling (PAC) randomized controlled trial (RCT), which aims to assess the impact of integrating a PA counsellor into a primary care practice. This RCT has 2 arms: (i) brief PA counselling (2-4 min) from a health care provider and (ii) brief PA counselling+intensive PA counselling from a PA counsellor (3 months). The impact of this intervention is being evaluated using the comprehensive RE-AIM framework. One hundred twenty insufficiently active adult patients, aged 18 to 69 y and recruited during regular primary care visits have been randomized. Dependent measures include psychological mediators, PA participation, quality of life, and physical and metabolic outcomes. The PAC project represents an innovative, theoretically-based approach to promoting PA in primary care, focusing on psychological mediators of change. We anticipate that key lessons from this study will be useful for shaping future public health interventions, theories, and research.