To study the cardiac effects of running, 15 healthy non-athletic male volunteers underwent a moderate training schedule for six weeks; thereafter five continued to run at a similar intensity for a further six weeks and 10 stopped. Left ventricular wall thickness and dimension, relaxation, and diastolic function were studied by digitised echocardiography at entry and at two, four, and six weeks, and again at 12 weeks. Significant increases in running ability and maximum oxygen consumption were observed. Maximum oxygen consumption was unchanged in those who continued to run but a reduction was noted in those who stopped. No significant changes in left ventricular dimensions were observed, but the thickness of the posterior wall and the septum increased during six weeks' running. Left ventricular mass (cube volume formula) increased significantly during the first six weeks. Comparison of those who continued to train and those who stopped showed that the former had no change in mass, whereas in the latter mass returned towards pre-exercise values. Systolic function (fractional shortening) was unchanged throughout the study. In addition, no abnormalities of relaxation or diastolic function were detected. Exercise induced left ventricular hypertrophy may develop rapidly during training and the increased myocardial mass resulting from six or 12 weeks' running is not associated with impaired relaxation and diastolic function as found in pathological forms of hypertrophy.