The National Exercise and Heart Disease Project (NEHDP) was funded by the Rehabilitation Services Administration (RSA) of the Department of Health, Education and Welfare, to determine the effects of regular physical activity on the rehabilitation, morbidity and mortality of patients with healed myocardial infarctions. Planning and development lasted from June 1972 through September 1974. Since 1974, 932 subjects were referred for evaluation. At randomization, 651 subjects were assigned to exercise treatment (323) or control (328) groups. Those who qualified for randomization had to complete an initial evaluation, attend 14 of 18 consecutively scheduled, low-level physical activity sessions during a period of 6 weeks, and complete a second evaluation. This prerandomization phase was accompanied by significant alterations in work capacity, heart rate levels at rest and during three levels of physical stress, systolic blood pressure reductions during stress but not at rest, and by changes in the level of anxiety and depression. The subjects will be followed for a minimum of 2 years at regular intervals to determine if regularly performed physical activity is beneficial to the rehabilitation of myocardial infarction survivors.