The effects of a period of 4 weeks training on treadmill exercise performance were evaluated in 27 patients, mean +/- SD age 55 +/- 7 years, who had previously suffered a myocardial infarction (17 Q-wave) 3-24 months (mean 11 +/- 8). To ensure comparability of exercise levels only patients who obtained their predicted maximal heart rate in the initial test were included. All trained 3-4 hours daily 5 days a week at an average maximal intensity of 85% of their initial peak heart rate. They performed 2 maximal exercise tests before and 1 after the training course. Maximal oxygen consumption was 28.1 +/- 5.3 and 28.8 +/- 6.5 ml/kg/minute (NS) before, and increased by 16% to 33.4 +/- 7.2 after training (P less than 0.01). Treadmill exercise distance was 510 +/- 153 and 559 +/- 163 meters (10% increase, P less than 0.01) before, and increased by 14% to 638 +/- 156 after (P less than 0.01). Heart rate, ratio of respiratory gas exchange, and breathing frequency remained unchanged in all three tests at maximal exercise, but were significantly lower at identical submaximal levels after training, while the respiratory tidal volume increased. Resting heart rate decreased by 12% after (P less than 0.01). Thus, aerobic exercise performance is improved by short-term training after myocardial infarction. By the longitudinal design of the study, and the maximal initial exercise test, this physiological improvement can be differentiated from that of increased motivation, and of increased treadmill exercise distance due to improved exercise technique.