Abstract: Experiments on the pyrimidine metabolism in Schistosoma japonicum have demonstrated that labelled bicarbonate, orotic acid and uracil are all incorporated into the nucleic acids of worms. This indicates that carbamyl phosphate synthetase II-aspartate carbamyl transferase-dihydroorotase complex, dihydroorotate dehydrogenase as well as orotate phosphoribosyl-transferase-orotidylate decarboxylase complex do exist.S. japonicum not only possesses all the enzymes of the pyrimidine de novo pathway but also the capacity of salvaging the preformed pyrimidines. It is evident that host blood may supply sufficient amount of bicarbonate to meet the requirements of de novo synthesis, but it is questionable whether blood contains enough quantity of pyrimidine bases and nucleosides for the salvage pathway.The effect of schistosomicides (niridazole and amoscanate) on the pyrimidine de novo pathway in vivo has also been studied. It is suggested that the orotate phosphoribosyl-transferase-orotidylate decarboxylase complex is a vulnerable point which might be attacked by chemotherapeutic agents. Since the structure of niridazole is similar to that of orotic acid, analogues of orotic acid may be of selective significance.