Abstract: Mice infected with 500-1000 cercariae each were treated intragastrically with pyquiton at a single dose of 300 mg/kg 2h before or 3 and 7 days after the infection. The respective histochemical changes of schistosomule of S. japonicum in mouse skin, lung and liver were compared 2-72 h after treatment.The activity of AKP in parenchymal cells and the glycogen content in somatic musculature and parenchymal tissue of schistosomules in skin were markedly decreased 2h after treatment. 6-72h after drug administration, both glycogen and AKP disappeared in some schistosomules. Both the AKP activity and the glycogen content of schistosomules in lung and liver showed slight changes and recovered quickly in a most all of the schistosomules. No marked change in the content and distribution of nucleic acid, alkaline protein, protein bound α-amino groups, protein bound tyrosine, tryptophan, histidine groups and in the activities of adenosine-triphosphatase and acid phosphatase was detected within 2-72h after treatment except in dead worms. WHO Collaborating Centre for Malaria, Schistosomiasis and Filariasis. Partial financial support was received from UNDP/World Bank/WHO TDR