Left ventricular function was examined during incremental work rates to maximum using simultaneous determinations of stroke volume, left ventricular ejection time (LVET), and diastolic filling time (DT). Seven endurance trained and seven untrained young adult males were studied on a cycle ergometer at matched heart rates of 90, 120, 140, 160, 180, and 190 bpm (+/- 1 bpm). Stroke volume of the untrained subjects reached a plateau at 120 bpm, but stroke volume of the trained subjects continued to increase to their maximum heart rate with no plateau. Throughout incremental work rates, LVET was significantly longer and DT was significantly shorter in the trained subjects. At a heart rate of 190 bpm, the corresponding rates of ventricular emptying and ventricular filling of the trained versus the untrained subjects were 20% and 71% greater, respectively (P < 0.01). We conclude that during incremental work rates the stroke volume of endurance trained athletes increases progressively to maximum with no plateau. In addition, although trained athletes rely on enhancements in both ventricular filling and ventricular emptying to augment stroke volume, by far their major advantage over untrained subjects is in ventricular filling.