We examined the hypothesis that a decline in Na-K pump activity contributes to the lower resting metabolic rate (RMR) in older males, independent of the loss of fat-free mass. Plasma and erythrocyte Na and K concentrations were measured using flame photometry. Changes in these concentrations after incubation with and without ouabain were used to calculate erythrocyte Na-K pump rate and rate constant in 27 younger (28 +/- 7 yr) and 25 older (67 +/- 6 yr) men. Older men showed an 18% lower erythrocyte Na-K pump rate constant (0.32 +/- 0.09 vs. 0.39 +/- 0.10 h-1; P < 0.01), a 12% lower RMR (1.10 +/- 0.14 vs. 1.25 +/- 0.14 kcal/min; P < 0.01) and an 8% lower level of fat-free mass (61.7 +/- 5.8 vs. 67.4 +/- 8.1 kg; P < 0.01) relative to younger men. A lower RMR persisted in older men (1.14 +/- 0.12 kcal/min) compared to younger men (1.21 +/- 0.12 kcal/min; P < 0.05) after control for the effect of fat-free mass. No differences in RMR were found, however, between older (1.17 +/- 0.13 kcal/min) and younger men (1.20 +/- 0.13 kcal/min) after controlling for both fat-free mass and the erythrocyte Na-K pump rate constant. A positive relation was noted between RMR and the erythrocyte Na-K pump rate constant, after removing the effects of fat-free mass (partial r = 0.30; P < 0.05). Our results support the conclusions that: 1) the in vivo activity of the Na-K pump is related to RMR, and 2) the age-related reduction in Na-K pump activity is a partial contributor to the decline in RMR in older men.