1. Methods are described for study of metabolism of human skeletal muscle in situ , at rest and during mild sustained contraction in the fed and fasted states. 2. At rest the average oxygen uptake was 0.29 ml min −1 100 ml of muscle −1 and the carbon dioxide output was 0.22 ml. Glucose uptake was 0.49 mg min −1 100 ml of muscle −1 . The respiratory quotient was 0.75, indicating that most of the glucose was being stored. 3. When subjects made hand-grips of 5% of their maximal voluntary contraction force (5% MVC) the oxygen and carbon dioxide exchanges both increased by six times while the glucose uptake increased by 70% of the resting value. 4. A 7 h fast before the observations were made severely decreased both resting and exercising glucose uptake but produced no other alteration in the metabolism of the muscle.