1541 Resting metabolic rate (RMR) declines with age in adult humans independent of reductions in fat-free mass. Habitual endurance exercise is associated with elevations in RMR. The physiological mechanisms responsible for these age- and exercise- effects on RMR are unknown. We tested the hypothesis that β-adrenergically-mediated sympathetic nervous system (SNS) support of RMR is greater in older endurance-trained compared with sedentary adults. RMR was measured (indirect calorimetry, ventilated hood) before and during complete, non-selective β-adrenoceptor blockade (i.v. propranolol) in 5 sedentary adults (2M/3F; 65 ± 3 yr; 31 ± 4% body fat; VO2max 26.8 ± 2.7 ml·kg−1·min−1) and 7 endurance-trained runners (5M/2F; 69 ± 1 yr.; 20 ± 2% body fat; VO2max 36.0 ± 2.9 ml·kg−1·min−1). RMR was significantly higher in the endurance-trained compared with the sedentary subjects (1349 ± 69 vs 1190 ± 71 kcal·d−1, p<0.01). Consistent with our hypothesis, the decrease in RMR in response to β-blockade was greater in the trained subjects when expressed either as an absolute (−70 ± 25 vs 19 ± 15 kcal·d−1, p=0.05) or a relative (−5.5 ± 1.9 vs 1.6 ± 1.2%, p<0.05) change. These data indicate that tonically-augmented sympathetic β-adrenergic stimulation of energy metabolism may contribute to the greater RMR of older adults who exercise regularly. Supported by: NIH AG13038, AG06537, AG00828, DK07658