Female Sprague-Dawley rats made chronically uremic by ligation of approximately three-fourths of the left renal artery and contralateral nephrectomy were shown to have a decreased ratio of tyrosine to phenylalanine in plasma. To ascertain whether this abnormal ratio was related to alterations in metabolism of phenylalanine or tyrosine, phenylalanine hydroxylase and tyrosine aminotransferase activities were measured in uremic and pair-fed sham-operated control rats. In the liver, activities of both phenylalanine hydroxylase and tyrosine aminotransferase were similar in uremic and pair-fed control animals. However in the kidney, the activity of phenylalanine hydroxylase, expressed either per gram of tissue or per total organ mass, was reduced in uremic rats as compared with that of controls. These observations suggest that the decreased phenylalanine hydroxylase activity of the kidney may contribute to the abnormal ratio of tyrosine to phenylalanine found in plasma of chronically uremic rats.