Certain metabolic changes are known to occur in response to surgical trauma. An understanding of these changes is of importance for proper handling of patients during surgery and convalescence. Numerous experimental and clinical studies on various aspects of metabolic response to surgery have been published during recent years, but relatively little work has been done on possible alterations in lipid metabolism as a result of operative trauma. Surgery may be associated with a significant loss of body fat. This is believed to be due to mobilization of depot fat stores for energy. After major surgery some patients are bound to subsist on a relatively deficient caloric intake for a considerable length of time. Intravenous fat emulsions have come into use as a means of supplementing caloric intake in such patients. But, what is known about the state of fat metabolism in a patient undergoing surgery? Is the patient capable of