Changes in blood volume (BV) and hemoglobin concentration ( [Hb] ) were induced in five healthy young men. After acute hypovolemic anemia was achieved by blood withdrawal, VO2max values decreased, while the same [Hb] due to acute plasma volume expansion (6% dextran) did not alter VO2max. After reinfusion of red blood cells, leading to hypervolemia and increased [Hb], VO2max increased. Plasma volume expansion in this situation, leading to hypervolemia at normal [Hb], resulted in a slight reduction in VO2max, which, however, remained elevated (approximately 4%) above control values. Physical performance, measured as time to exhaustion, corresponded to the changes in VO2max except for the hypervolemic anemic situation, where it decreased. Changes in peak heart rate were inversely related to BV changes, but were also influenced by [Hb]. The results point to a significant influence of the total amount of Hb rather than the blood hemoglobin concentration for obtaining a high maximal aerobic power. Thus, a reduced [Hb], concomitantly with an elevated blood volume (plasma volume), may result in an unchanged VO2max, but reduced performance time.