1. The exchange of plasma amino acids and glucose, lactate, glycerol and 3-hydroxybutyrate in the leg was studied in otherwise healthy patients undergoing elective cholecystectomy. 2. Seven patients were given a constant intravenous infusion of glucose at a rate of 1.1 mmol/min throughout the study. Seven other patients who received normal saline only served as a control group. 3. Measurement of leg blood flow and arterio-femoral venous differences of amino acids and other energy metabolites were made on four occasions: (I) before surgery, (II) 10 min after skin incision, (III) at the end of surgery, and (IV) 30 min after the end of anaesthesia. 4. The release of amino acids from the leg was comparable in the two groups before and during the early part of surgery. At the end of surgery the release of several individual amino acids, as well as the total release of amino acids, from the leg was higher in the patients given glucose infusion compared with the control patients. 5. The infusion of glucose prevented the intraoperative rise in arterial levels and uptake of 3-hydroxybutyrate in the leg. A high release of amino acids at the end of surgery was thus associated with low arterial levels of 3-hydroxybutyrate while the reverse pattern was seen in the control patients. 6. These effects of glucose infusion were qualitatively different from those seen in uninjured post-absorptive man.