This study examined the effects of short-term endurance training (ET) on the left ventricular adaptation and functional response to a series of exercise challenges with increasing intensity. Eight untrained males with a mean age=19.4±0.5 years (± sem) were studied before and after six days of ET consisting of cycling two hr/day at 65% peak aerobic power (VO_2max). Left ventricular (LV) ejection fraction (EF), and LV volumes were assessed by radionuclide angiography at rest, and during exercise at 3 uninterrupted successive workrates corresponding to 53%, 68% and 83% of VO_2max, each lasting 20 minutes. ET produced a calculated plasma volume (PV) expansion of 11.4±2.2% (p<0.05). The increase in PV was accompanied by an increase in VO_2max from 45.9±1.9 to 49.0±1.0 ml/kg/min (p<0.01) and a decrease in maximal heart rate (HR;197±2.3 to 188±1.0 beats/min; p<0.01). Resting LV function was not changed, although there was a trend for higher SVs and improvement in the rapid filling phase of diastole (p=.08). Training induced an increase in exercise SV by 10.4%, 10.2% and 7%, at 53%, 68% and 83% VO_2max, respectively (p<0.01). These changes were secondary to increases in end-diastolic volume, which increased significantly at each exercise workrate following training (139 ± 6 to 154±6 ml at 53%VO_2max, and from 136±5 ml to 156±5 ml at 83% VO_2max (p<.01). End-systolic volumes were unchanged after ET. A significant bradycardia was observed both at rest (decreasing 7%) and exercise (decreasing 10.4%). LVEF during exercise was increased slightly by training, reaching significance at the highest workrate, after 60 minutes of exercise. (p<.05). Cardiac output was higher following training at the highest workload (20.8±2.2 vs. 22.9±3.1 l/min; p<.01). These data indicate that short-term training elicits rapid adaptation to the LV functional response exercise, with increases in SV being secondary to a Frank-Starling effect with minor changes in contractile performance. This produced a volume-induced bradycardia and increase in LV filling which may be of benefit during prolonged exercise.