Five patients were studied before and two to three days after major, uncomplicated abdominal operation. Muscle tissue was obtained by needle biopsy from m. quadriceps femoris after eight hours overnight fast. Plasma free amino acids were analyzed in simultaneously obtained samples. In the homogenized muscle samples the intracellular concentration of each amino acid (IC) was calculated by subtracting the free extracellular part from the total amount, assuming the plasma concentration to be equal to the concentration in the interstitial fluid. Their relationships have also been calculated (IC/EC gradient). The extra- and intracellular water distribution was estimated using a modified chloride method. In similarity to the findings in normal subjects the majority of the amino acids showed much higher concentration in intracellular water than in plasma. Preoperatively all amino acids examined in muscle biopsies were formed within normal limits. Postoperatively the total amount of free amino acids in plasma and muscle was decreased, and the amino acid profiles differed from those observed in normal subjects. In plasma, as compared with normal controls, the most significant changes were an increase in phenylalanine and tyrosine and a decrease in serine, proline, histidine and isoleucine. In muscle the greatest decrease occurred in the concentrations of glutamine, arginine and lysine followed by proline and glutamic acid. The increase in taurine, valine and phenylalanine were all highly significant and in serin, glycine, alanine and leucine significant, whereas tyrosine showed only a moderate rise. Compared with normal values there were marked increases in the gradient between intracellular and plasma concentrations which were highly significant for glycine and valine and significant for serine, alanine, isoleucine and leucine. The shift in the methionine gradient was somewhat less. We confirm that alterations in the muscle free amino acid pool are not reflected in the values found in plasma. Further work is required to explore the clinical significance of the observed variations in individual amino acids.