Twenty patients were studied over the first 4 post-operative days following abdominal aortic surgery. Ten patients had 93% of their non-protein energy as glucose and insulin was given to keep blood glucose below 10 mmol/l. The other 10 patients had 80% of non-protein energy as fat (Intralipid). Amino acids corresponding to 12 g of nitrogen were given in both groups. Gas exchange, nitrogen balance, phosphate balance, vanillylmandelic acid (VMA) excretion, 1- and 3-methylhistidine in urine, acute phase proteins, immunoglobulins and albumin were followed. Substrate utilisation was calculated from indirect calorimetry data and nitrogen excretion. Metabolism in the early post-operative phase was found to adapt to the nutrition regimen given even though the composition was extreme either in fat or carbohydrate content. The glucose-insulin regimen had a better nitrogen sparing effect and based on the difference in 3-methylhistidine excretion it is suggested that mainly protein muscle benefitted from this. Regardless of the TPN-regimen given, those patients whose RQ deviated the most from the average in their group had the highest nitrogen excretion. The two groups showed no differences in plasma proteins and catecholamine excretion.