We have examined in elderly patients the effect of maintenance of normothermia during hip surgery on postoperative protein metabolism. In one group of six patients (warmed group) heat loss was minimized during surgery and in the recovery period by warming fresh gases, i.v. fluids and wrapping the exposed parts of the body with a warming blanket. In a second group of six patients (cold group), routine care was provided. General anaesthesia consisted of thiopentone, tubocurarine and halothane in both groups. Urinary excretion of urea nitrogen and 3-methylhistidine (3-MeH) after surgery was significantly lower in the warmed group compared with the cold group (P less than 0.05). There was little effect of normothermia on amino acid concentrations in plasma after surgery. Muscle glutamine concentration 4 days after surgery decreased by 50% in the cold group and 18% in the warmed group. Total body potassium (TBK), measured as an index of body cell mass, decreased significantly after surgery in both groups. However, 7 days after surgery the reduction in TBK in the cold group remained significantly lower than that of the warmed group (P less than 0.05). Maintenance of normothermia during hip surgery appeared to attenuate, but not eliminate, protein breakdown and nitrogen loss after surgery.