We studied the effect of parenteral nutrition with amino acids and hypertonic glucose on protein synthesis and liver fat metabolism. Patients with operable gastrointestinal tract malignancies were divided into two groups. Group I ate the hospital diet ad libitum for the 7-10 days preceding surgery. Group II were given adjuvant parenteral nutrition (APN) for 7-10 days prior to the surgical removal of the tumor. Daily nutrient intake and nitrogen balance were determined. [15N[glycine (1-2 g) was infused at a constant rate for 12-18 prior to surgery. During surgery, blood, liver, and muscle specimens were taken for 15N analysis. Fractional protein synthesis rates were estimated by the method of Garlick et al. (Biochem. J. 136: 935-945, 1973). The fat content and distribution pattern in the liver was determined by gas chromatography-mass spectrometry. The following results were found. 1) APN increaed the albumin synthesis rate. 2) The fraction of linoleate in the total liver fatty acids were reduced by 75% in the APN patients. 3) Some of the APN patients developed fatty livers during the study. When the APN patients were subdivided on the basis of whether they had fatty livers, it was found that only those patients who did not accumulate fat showed an improvement in their plasma albumin concentration during the period of parenteral nutrition.