Pyrimidine synthesis and its regulation are presented. Among the disorders of human pyrimidine metabolism, hereditary orotic aciduria and that produced by drugs play the principal role. A rise in renal excretion of orotic acis is also observed when ornithine transcarbamylase activity is lacking. The importance of "orotic aciduria with partial response to folic acid" in pyrimidine metabolism is still not clear. Close relationship between the formation of pyrimidine and purine nucleotides must be assumed, because both enter into the synthesis of nucleic acid, for the greatest part in approximately equimolecular amounts. Possibly 5-phosphoribosyl-1-pyrophosphate plays an important part.