Since the 1950's a considerable number of published reports have implicated sedentary living as one of the number of risk factors associated with the mid-20th Century increase in mortality from ischaemic heart disease or, specifically, from two of its critical manifestations, i.e., myocardial infarction and sudden cardiac death. The advantage in mortality is consistently reported as being in the order of two to threefold in favour of the physically active. Conclusive proof of the protective value of exercise can only be obtained by means of a large prospective study, the size and duration of which would now be far too costly to undertake. Our only course, then, is to examine the circumstantial evidence available, including the possible mechanisms by which regular physical exercise could benefit the cardiovascular system. Animal, and more recently human, studies have shown that endurance-training can bring about morphological, biochemical, and vascular changes in the heart, the net effect being improved dynamic performance. Peripheral changes also contribute to the development of a more efficient oxygen transport system. In those who suffer from ischaemic heart disease, exercise has been shown to reduce symptoms, alleviate depression, improve plasma lipid profile, and render the cardiovascular system less susceptible to stress. While non-randomised trials have indicated that supervised training improves prognosis following myocardial infarction, randomised trials, for a variety of reasons, have given inconclusive results. On the other hand, physician-supervised exercise has been shown to be a safe therapeutic intervention, and in view of its proven psychological and physiological benefits it has assumed major public health importance.