1. Under stress an increased amount of labelled amino acids is incorporated. 2. Under the conditions of stress amino acids too participate in the process of synthesis. 15N-glycine was used as tracer in a quantity of 0.64 g of amino acid and 24 kcal per kg per day, that is at a 0.027 g per kcal. hour infusion rate, so that the infused 15N corresponded to 75% of the total residual nitrogen in healthy subjects and 57% in operated patients. 3. Sixteen percent of the infused 15N is secreted with the urine of healthy subjects as urine nitrogen, and 57% as urine nitrogen and 4--5% as ammonia nitrogen with the urine of the operated cases. 4. In contrast to normal conditions, under stress a higher percentage of the infused amino acids participates in energy production. 5. Stress enhances uropoiesis as in regards both quantity and speed. 6. No difference in acid-base balance was found in the case of postoperative infusion; urinary and plasma concentrations remained within normal limits.