This study was conducted to resolve discrepancies in the literature with regard to changes in protein metabolism following surgical stress. Twelve patients who had undergone abdominal surgery and six who were controls were studied. Whole body protein turnover was measured on the third and 10th postoperative day, during isonitrogenous and isocaloric total parenteral nutrition (TPN), by the method of constant infusion of [15N]glycine. Six patients who underwent abdominal surgery without any complications showed positive nitrogen balance on the 10th postoperative day (group I). However, nitrogen balance was still negative on the 10th postoperative day in another six patients who showed some critical complications after abdominal surgery (group II). A significant increase in whole body protein breakdown was seen in groups I and II on the third postoperative day (p less than 0.02, p less than 0.01, respectively), compared with control. Breakdown was greater in group II (p less than 0.05) than in group I. Protein synthesis tended to increase in group II, whereas in group I, it was at the same level as control. It was concluded that protein synthesis was unchanged in moderate stress, concomitant with increment of breakdown, but it tended to increase with a greater increase of breakdown in severe stress.