The occurrence of low tyrosine tissue levels in uremic subjects, possibly due to impaired phenylalanine hydroxylation, suggests that tyrosine may be an essential amino acid in uremia. Additional dietary tyrosine may thus re-dress the deficiency. This study examined growth and tyrosine/phenylalanine metabolism in uremic rats during tyrosine supplementation. Rats made uremic (U) by 7/8 nephrectomy were compared to pair-fed (CP) and ad libitum-fed (CA), sham-operated controls. Two sets of each group of rats were studied after 21 days on the respective diets: I = Purina Lab Chow; II = same + 3.5% tyrosine. Plasma tyrosine was below normal in U and CP-fed diet I. With diet II, the tyrosine:phenylalanine ratio in U was lower than both CA and CP. In rats fed diet II, the tyrosine:phenylalanine ratio became indistinguishable among the three groups. Growth parameters in U and CP were similar, regardless of the diet. Body weight gain, tibial length, muscle mass, and tissue protein did not improve in uremic animals supplemented with tyrosine. The specific activity of liver phenylalanine hydroxylase in U was not different from CA or CP. However, loss of cortical renal mass appeared to be the major determinant of decreased kidney phenylalanine hydroxylation in experimental uremia. This alteration is likely to be the greatest contributory factor to the alteration of plasma levels of tyrosine and phenylalanine. The data presented do not support a proposed essentiality of tyrosine in uremia.