To clarify the mechanism responsible for the increase in stroke volume (SV) due to training, we investigated the effects of interval training on the left ventricle using M-mode echocardiography. Six healthy male subjects volunteered to undergo 48 training sessions for 12 weeks (4 sessions· week^-1) One session consisted of five periods of exercise of 3-min duration on a cycle ergometer at a power output of 100% maximal O₂ uptake (Vo₂max), interspersed with 2-min recovery cycling at 50%Vo₂max. The echocardiograms at rest and during mild exercise (100W) were recorded before and after the training. The interval training significantly increased Vo₂max. Although there was no significant difference in SV at rest before and after the training, the training increased SV significantly during exercise. Before the training, there was a significant difference in left ventricular enddiastolic dimension (LVEDD) and left ventricular end-diastolic volume (LVEDV) at rest and during exercise. However, after the training, LVEDD and LVEDV during exercise were significantly larger than those at rest. These results suggest that interval training for 12 weeks increases diastolic filling (elasticity) of the left ventricle during exercise in healthy young men, partly contributing to the increase in SV due to the training.