The effects of abdominal operation on the antecubital vein plasma amino acid concentration were examined. With four groups, each containing ten patients, factors such as severity of surgical operation, anesthesia, and postoperative nutrition were examined. Immediately after operation there is a fall in the plasma concentrations of most amino acids; the nonessential amino acids (glutamate, proline, glycine, alanine, histidine, and arginine) continue to fall during the 2 postoperative days, whereas the essential amino acids (isoleucine, leucine, phenylalanine, and lysine, with tyrosine) are at higher concentration at 48 hours than immediately after operation. Comparison of a group of patients undergoing an operation of moderate severity (vagotomy and pyloroplasty or cholecystectomy) with those having the more severe procedure of resection of aortic aneurysm with Dacron graft replacement suggests that these general changes in amino acid concentrations do not appear to be due to the severity of the operation, although cystine levels were lower, and phenylalanine levels were higher, in the graft-replacement group. The changes did not appear to be the result of anesthesia. Increasing the postoperative glucose intake was associated with higher plasma alanine and lower methionine levels. It is suggested that, in surgical patients, cystine and tyrosine may become essential amino acids and rises in phenylalanine and methionine indicate transient liver dysfunction. The data give support to the view that a high calorie intake after operation has a beneficial effect.