We tested the hypothesis that resting metabolic rate (RMR) declines with age in physically active men (endurance exercise 3 times/wk) and that this decline is related to weekly exercise volume (h/wk) and/or daily energy intake. Accordingly, we studied 137 healthy adult men who had been weight stable for 6 mo: 32 young [26 ± 1 (SE) yr] and 34 older (62 ± 1 yr) sedentary males (internal controls); and 39 young (27 ± 1 yr) and 32 older (63 ± 2 yr) physically active males (regular endurance exercise). RMR was measured by indirect calorimetry (ventilated hood system) after an overnight fast and ~24 h after exercise. Because RMR is related to fat-free mass (FFM; r = 0.76, P < 0.001, current study), FFM was covaried to adjust RMR (RMR_adj). RMR_adj was lower with age in both the sedentary (72.0 ± 2.0 vs. 64.0 ± 1.3 kcal/h, P < 0.01) and the physically active (76.6 ± 1.1 vs. 67.9 ± 1.2 kcal/h, P < 0.01) males. In the physically active men, RMR_adj was related to both exercise volume (no. of h/wk, regardless of intensity; r = 0.56, P < 0.001) and estimated energy intake (r = 0.58, P < 0.001). Consistent with these relations, RMR_adj was not significantly different in subgroups of young and older physically active men matched either for exercise volume (h/wk; n = 11 each) or estimated energy intake (kcal/day; n = 6 each). These results indicate that 1) RMR, per unit FFM, declines with age in highly physically active men; and 2) this decline is related to age-associated reductions in exercise volume and energy intake and does not occur in men who maintain exercise volume and/or energy intake at a level similar to that of young physically active men.