Praziquantel (EMBAY 8440, Droncit) is a new type of acylated isoquinoline-pyrazine. A single oral or subcutaneous dose of the compound is reliably effective against juvenile and adult cestodes in mice and rats. For the first time, with praziquantel a compound is available that is also effective on cestodes in the bile-duct. Using Hymenolepis nana in mice we were able to show that age, sex, strain and intensity of infestation of the host have no influence on the efficacy. The onset of the effect of praziquantel in vivo is rapid. Within 10 min the parasites were immobilized and contracted and excreted within a few hours with the faeces.