To evaluate the effect of exercise on left ventricular diastolic filling, the following were measured at rest and during exercise in 14 control subjects and 15 athletes, using digitized M-mode echocardiography: the peak early diastolic lengthening rate of the left ventricular dimension and the filling volume and the filling fraction during the first 0.10 s of diastole. During ergometer exercise performed at a level that increased the heart rate to 100 beats/min, there were significant increases in the peak normalized lengthening rate of the left ventricular dimension (control subjects, 4.2 +/- 1.3 vs. 6.1 +/- 1.1 s-1, mean +/- SD, P less than 0.001; athletes, 5.3 +/- 0.9 vs. 7.4 +/- 1.1 s-1, P less than 0.001), filling volume (control subjects, 15 +/- 12 vs. 33 +/- 10 ml, P less than 0.001; athletes, 21 +/- 12 vs. 63 +/- 18 ml, P less than 0.001), and filling fraction (control subjects, 21 +/- 14 vs. 42 +/- 17%, P less than 0.005; athletes, 21 +/- 13 vs. 54 +/- 12%, P less than 0.01). The peak lengthening rate of the left ventricular dimension, the filling volume, and the filling fraction were significantly greater in athletes than in control subjects during exercise (P less than 0.005, P less than 0.001, and P less than 0.05, respectively). Augmented early diastolic filling may be a mechanism to provide adequate filling for the ventricle at high heart rates produced by exercise, especially in athletes.