MCELVANEY, G. N., S. P. BLACKIE, N. J. MORRISON, M. S. FAIRBARN, P. G. WILCOX, and R. L. PARDY. Cardiac output at rest and in exercise in elderly subjects.Med. Sci. Sports Exerc, Vol. 21, No. 3, pp. 293–298, 1989. We measured cardiac output (&OV0422;), at rest and during graded exercise, in 68 women and 41 men over the age of 55 yr, using a CO2rebreathing method. Mean (±SD) age was 66±5 yr in women and 66±6 yr in men. Only subjects with no history or physical examination findings of pulmonary, cardiac, neuromuscular, or endocrine disease and normal electrocardiograph and spirometry were studied. We found a linear relationship between &OV0422; and oxygen uptake (&OV0312;O2) in males and females. The regression equation expressing this relationship in males was &OV0422; = 2.9 + 5 &OV0312;O21·min-1(SEE 2.8) and, in females, &OV0422; = 2.9 + 4.6 &OV0312;O21·min-1(SEE 2.8). This is similar to the relationship previously estimated for elderly males using the direct Fick method and concurs with other reports in the literature which show that, while the &OV0422;-&OV0312;O2relationship in the elderly has a slope similar to that in younger groups, the &OV0422;-&OV0312;O2intercept is lower. This means that the absolute level of cardiac output for a given level of work is lower in the elderly than in younger populations. This may reflect an age-related decrease in active metabolic tissue in the elderly and/or altered metabolic regulation with increased oxygen extraction from blood.