Repeated estimations of circulating total lipids and various fractions were made during the postoperative period in patients receiving parenteral nutrition for more than ten days. The patients were paired in relation to the underlying pathology and divided into two group : the first receiving energy supplements in the form of carbohydrates alone, whilst the second received part of this supply in the form of lipid emulsions (Trive 1000). Total lipids and various plasma fractions, with the exception of free fatty acids, increased progressively during the postoperative period, regardless of the inital values and independently of the lipid content of the nutrition fluids given. There was no significant difference between those patients who received lipid emulsions and those who did not. Free fatty acids remained at levels slightly greater than normal in the group which did not receive lipids. They were significantly higher in the group given lipid emulsions, though it was not possible to precisely define their orgin.