The primary aim of this investigation was to test the general hypothesis that aging does not influence total blood volume in humans. To accomplish this, we measured supine resting plasma volume (modified Evans blue dye technique) and hematocrit in healthy nonobese young (24.7 +/- 1.2 yr, n = 7) and older (66.1 +/- 1.8 yr, n = 7) men carefully matched for body size and daily energy expenditure. Estimated body fat was greater (23 +/- 2 vs. 17 +/- 1%, P < 0.01) and maximal O2 consumption was lower (32.4 +/- 2.1 vs. 48.7 +/- 1.5 ml.kg-1.min-1, P < 0.001) in the older men than in the young controls. Absolute levels of total blood volume were 24% lower (P < 0.01) in the older men than in the young controls (4,749 +/- 333 vs. 6,234 +/- 318 ml). The lower absolute level of total blood volume in the older men was due to a 21% lower (P < 0.01) plasma volume (2,956 +/- 194 vs. 3,745 +/- 176 ml) and a 28% lower (P < 0.01) erythrocyte volume (1,792 +/- 148 vs. 2,488 +/- 158 ml) than in the young controls. Total blood volume, plasma volume, and erythrocyte volume also were lower (P < 0.01) in the older than in the young men when expressed relative to body weight, body surface area, or estimated fat-free mass. These findings suggest that total blood volume decreases with age in healthy men of similar size and chronic physical activity levels.