Metabolic changes in 10 patients undergoing total hip replacement were studied. Metabolic expenditure postoperatively remained within 5–10% of preoperative values. Net nitrogen loss for the postoperative period was less than 35 gm for the first 4 postoperative days, well below changes seen in major nonsurgical trauma. Fluid balance was positive in all patients for the first 4 postoperative days. Patients not in negative fluid balance by the fourth postoperative day should be closely monitored for complications. Recognition of delayed fluid excretion is essential in order to avoid the complications of further fluid loading in the postoperative period. Muscle composition reveals predictable changes in extracellular water, sodium, and chloride composition following surgery.Our data showed that metabolic changes following total hip replacement are in the order of major general surgical procedures and well below that observed with major trauma.