The role of skeletal muscle mass in the age-associated decline in maximal O2 uptake (VO2max) is poorly defined because of confounding changes in muscle oxidative capacity and in body fat and the difficulty of quantifying active muscle mass during exercise. We attempted to clarify these issues by examining the relationship between several indexes of muscle mass, as estimated by using dual-energy X-ray absorptiometry and treadmill VO2max in 32 chronically endurance-trained subjects from four groups (n = 8/group): young men (20-30 yr), older men (56-72 yr), young women (19-31 yr), and older women (51-72 yr). VO2max per kilogram body mass was 26 and 22% lower in the older men (45.9 vs. 62.0 ml.kg-1.min-1) and older women (40.0 vs. 51.5 ml.kg-1.min-1). These age differences were reduced to 14 and 13%, respectively, when VO2max was expressed per kilogram of appendicular muscle. When appropriately adjusted for age and gender differences in appendicular muscle mass by analysis of covariance, whole body VO2max was 0.50 +/- 0.09 l/min less (P < 0.001) in the older subjects. This effect was similar in both genders. These findings suggest that the reduced VO2max seen in highly trained older men and women relative to their younger counterparts is due, in part, to a reduced aerobic capacity per kilogram of active muscle independent of age-associated changes in body composition, i.e., replacement of muscle tissue by fat. Because skeletal muscle adaptations to endurance training can be well maintained in older subjects, the reduced aerobic capacity per kilogram of muscle likely results from age-associated reductions in maximal O2 delivery (cardiac output and/or muscle blood flow).