Fasting and 1 - 2-, 3- and 4-hour plasma phenylalanine levels following 50 mg/kg oral phenylalanine administration were increased above normal values in uremic patients. Plasma tyrosine concentrations, however, were not significantly different from normal values. The rate of plasma disappearance of phenylalanine was significantly longer in nondialyzed patients when compared to normal subjects and dialyzed uremics. These observations indicate that phenylalanine metabolism is altered in uremia.