- This paper reports the results of a study undertaken to explain levels of implementation of heart health promotion activities observed in Ontario public health agencies in 1997. Organizational-level data were collected by surveying all 42 health departments in 1994, 1996 and 1997 as part of the Canadian Heart Health Initiative Ontario Project. Guided by social ecological and organizational theories, the model examines relationships between implementation and four sets of possible determinants of activity: (1) the predisposition of agencies to undertake heart health promotion activities, (2) their capacity to undertake these activities, (3) internal organizational factors and (4) external system factors. A small set of five variables explains almost half of the variance in implementation (R2 = 0.46): organizational capacity (beta = 0.40), priority given to heart health (beta = 0.36), coordination of programs (beta = 0.19), use of resource centers (beta = 0.12) and participation in networks (beta = 0.09). The results suggest that models integrating organizational and socio-ecological theories can help us understand the implementation of community-based heart health promotion activities by public health agencies. Implications for future research, policy and practice are discussed.