Allopurinol-induced orotaciduria is reduced by dietary ribonucleic acid (RNA), RNA hydrolysate and different nucleotides. These findings are compatible with feedback regulation of pyrimidine biosynthesis by dietary nucleotides. Serum uric acid and urinary uric acid excretion on a purine-free isoenergetic diet reach a minimum after about 10 days and remain constant thereafter. When purines from different biochemical sources are added to such a diet there is always a linear relationship between dietary purines and serum uric acid level and urinary uric acid excretion. The findings suggest that dietary purines play a minor role if any in the regulation of purine biosynthesis in man.