Concurrent with the study of performance and aerobic power of 12 highly trained, superior middle-distance runners before, during and after 3 weeks of strenuous training at 2,300-m altitude, observations were made pre- and post-altitude on blood volume, BV, plasma volume, PV, cell volume, CV, hematocrit, Hctv, and the concentration and amount of hemoglobin, Hb. For comparative purposes, the same components were measured in 21 non-athletic young men at or near sea level. Similar data on 27 black and white sharecroppers also were available for comparison. Since CV did not change at altitude, Hctv values obtained on eight occasions at altitude were used to calculate PV at altitude. The mean Hctv increased 3.7% at altitude, which corresponds to a decrease or 6.6% in PV. The mean decrease in body weight, BW, was 2.1. In the runners BV in relation to BW was 21% greater than in 48 non-athletes. Approximately one-third of this larger BV is accounted for by the lower body fat of the runners. Cell volume, PV and total Hb, all in relation to BW, were greater in the runners than in the non-athletes by 18, 24 and 16%, respectively. Consistent with the difference in PV and in CV, Hb concentration was 4% lower in the runners. It is concluded that endurance athletes have thin blood but so much of it that their total Hb exceeds that of non-athletes. Aerobic power was dependent in part on Hb/kg not only in the runners but also in girl swimmers.