Effect of hemodialysis on phenylalanine distribution between the intracellular water of erythrocytes and the plasma obtained from patients with end-stage kidney was studied by fluorimetric measurements of phenylalanine concentration according to the method of Mc Caman and Robins. The measurements were performed twice: before and after 4 h of hemodialysis. It was found that the nondialyzed patients with end-stage kidney had higher than normal phenylalanine concentrations both in plasma and in the intracellular water of erythrocytes. Hemodialysis brought about a decrease in plasma phenylalanine level without influencing the erythrocyte concentration of the studied amino acid and without altering the function of the erythrocyte cell membrane, as far as the phenylalanine transport was concerned.