In 4 college athletes and 4 nonathle-tic freshmen measurements were made of ventilation, O2 consumption, cardiac output, and heart rate at 3 submaximal levels of exercise before, and again after, a period of athletic training. In both groups there was a decrease in heart rate, cardiac output, and minute ventilation at any given work load. Oxygen consumption was unaffected and therefore the arterial-venous O2 difference was increased. Before training, the athletes differed from the nonathletes in having a lower minute ventilation, a larger stroke volume at the 2 external work loads studied, and a slower heart rate at the higher load. These differences persisted after training, when it was found also that the athletes had lower values for cardiac output at equal exercise loads.