Maximal oxygen uptake (VO2max) determinations and haemodynamic studies at rest and during two submaximal exercise levels (60 and 85% of the maximal) were performed in 30 male patients under 60 yr of age, 2, 5 and 8 mth after an uncomplicated myocardial infarction. Fourteen patients participated in a physical training programme during the first 3 mth of the study, while 16 patients attended the training during the second 3-mth period. An increase in VO2max occurred at 5 mth in both groups, 16 and 11%, respectively. Corresponding to this improvement cardiac index (CI) at the same relative submaximal work load increased similarly in the two groups. During the second period of the study only slight increments in VO2max and no salient alterations of the haemodynamics were recorded within or between the two groups. The linear relationship between oxygen uptake and CI was not affected throughout the study. As the work load was augmented from 60 to 85% of the maximal during the exercise test, the stroke volume index did not tend to fall. Our data suggest that allowing all patients to resume work in the very early rehabilitation period after an uncomplicated myocardial infarction, the improvement in physical work capacity and cardiac function is not significantly affected by a training programme of low intensity and short weekly duration.