In an attempt to compare changes in erythrocyte count, hematocrit and erythropoietin levels of athletes induced by strenuous physical training with those of nonathletes, twenty three male well trained university athletes and 12 male nonathletic university students were subjected to exercise on a Monark bicycle ergometer with a working intensity of about 8 in relative metabolic rate for 2 hours for successive 7 days during summer. The results obtained were as follows: The mean values of decrease in erythrocyte count and hematocrit after one week's strenuous muscular exercise for non-athletes were greater than those for athletes. In both athletes and nonathletes, subjects with higher value of hematocrit showed a tendency of greater fall in hematocrit after physical training. Nonathletes showed smaller mean value of reticulocyte count before physical training than the athletes. The former showed an increase in reticulocyte count after physical training, while the latter showed essentially no change in reticulocyte count. Nonathletes showed lower erythropoietin levels before physical training than the athletes. Nonathletes showed no change in erythropoietin levels after physical training, while the athletes showed a marked increase in this variable after the training. These results suggest that there were marked differences between athletes and nonathletes in the state of destruction and production of erythrocyte before and after the strenuous physical training which lasted for 7 successive days. While the rate of erythrocyte production for nonathletes might be slower than that for athletes at any time, the rate of erythrocyte destruction for nonathletes might be slower before the physical training and might be faster during and after the physical training than that for the athletes.