A decrease in dietary protein intake lowers the clearance of a number of substances excreted principally by the kidney including uric acid and oxypurinol, the major metabolite of allopurinol. We studied the kinetics of uric acid and oxypurinol in seven healthy volunteers on a normal protein diet (2600 calories; 100 g protein) followed by a 400 calorie, protein‐free diet. A 600 mg dose of allopurinol was given orally after 6 days of the normal protein diet and again after 2 days of the 400 calorie, protein‐free diet. Two major findings emerged: first, the renal clearance of oxypurinol was reduced from 21.2 ± 1.9 ml/min during the normal protein diet to 12.3 ± 1.2 ml/min (P<.05) during the 400 calorie, protein‐free diet, and second, there was a striking diurnal difference in oxypurinol renal clearance with a 41% decrease in the oxypurinol clearance at night (8pmto 8am) versus day (8amto 8pm) on the 400 calorie, protein‐