The ideal type of an anthelmintic drug should be effective against the parasite and not toxic to the host. In early studies of this disease in Japan, a large number of different drugs was tested, none of which was promising enough. In 1956, an in vitro screening method for effective drugs against Paragonimus westermani was established. It was also found (1961) that rats experimentally infected with P. ohirai can be successfully employed for chemotherapeutic studies of paragonimiasis in place of dogs and cats infected with P. westermani. Thus, bithionol has been found to have a strong therapeutic effect against pulmonary paragonimiasis and is now used not only for individual treatment but also for mass treatment in endemic areas. However, it proved difficult to administer bithionol over 20 days because of reduced patient compliance, especially in patients of population based chemotherapy. To ensure good patient compliance and easy administration in such campaigns for disease control, treatment must be simple and given over a minimum period of time. Praziquantel (2-cyclohexylcarbonyl-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]++ +isoquinolin- 4-one, EMBAY 8440, Biltricide) is a recently introduced drug that has the unique characteristic of being effective in the treatment of infections with all schistosome species pathogenic to man and many other human trematode and cestode infections, and it has to be given on 1 to 2 days only. In this paper, the effects of praziquantel against Paragonimus in vitro using metacercariae of P. ohirai and rats experimentally infected with P. ohirai are described.