This study investigated the effect of fitness on arm perfusion and metabolism during upper body exercise. Eight average fit subjects and seven rowers, mean (± S.E.) maximal oxygen uptake (VO₂max) of 157 ± 7 and 223 ± 14 ml O₂ kg^-0.73 min^-1, respectively, performed incremental arm cranking exercise to exhaustion. Arm blood flow (ABF) was measured with thermodilution in v. axillaris and arm muscle mass was estimated by dual-energy X-ray absorptiometry. During maximal arm cranking pulmonary VO₂ was ~45% higher in the rowers compared with the untrained subjects and peak ABF was 6.44 ± 0.40 and 4.55 ± 0.26 l min^-1, respectively ( P< 0.05). The estimated arm muscle mass for the rowers and the untrained subjects was 3.5 ± 0.4 and 3.3± 0.1 kg, i.e., the arm perfusion was 1.9 ± 0.2 and 1.4 ± 0.1 l blood kg^-1 min^-1, respectively ( P< 0.05). The arterio-venous O₂ difference for the rowers and the untrained subjects was 156 ± 7 and 120 ± 8 ml l^-1, respectively, and, therefore, arm VO₂ was 0.98 ± 0.08 and 0.60 ± 0.04 l min^-1 corresponding to 281 ± 22 and 181 ± 12 ml kg^-1, while arm O₂ diffusional conductance was 49.9 ± 4.3 and 18.6 ± 3.2 ml min^-1 mmHg^-1, respectively (P< 0.05). However, lactate release in the rowers was almost thrice higher than in the untrained (26.4 ± 1.1 vs. 9.5 ± 0.4 mmol min^-1, P< 0.05). These data suggest that the energy requirements of the ~50% larger arm work capacity following long term arm endurance training are covered by a ~60% increase in aerobic metabolism and an almost tripling of the anaerobic capacity.