Summary Twelve anticoccidial or antimalarial drugs were tested for their efficacy against various developmental stages of Sarcocystis muris in NMRI-mice. Schizogonic stages present in the liver from day 11 to 17 p. i. showed to be most sensitive to drug action. Sulfaquinoxaline plus pyrimethamine, zoalene and Bay g 7183 completely eliminated these stages. A strong though not 100 percent efficacy was observed in experiments with primaquine. The other drugs tested were less (halofuginone, sulfadoxine plus trimethoprim) or not effective (sulfadimethoxine, amprolium, monensin, aprinocid, sulfaquinoxaline plus diaveridine) in the used dosages. In trials to improve the Sarcocystis muris-mouse-cat model it was found that in NMRI-mice the inoculation dose of 50 sporocysts resulted in the highest infection rate and intensity of the infection. By the application of less or more sporocysts or by repeated inoculations poorer infection rates and lower intensities of infection were achieved. Thymus deprived nude mice (NMRI-nu/nu) and the AKR/N-strain were the most susceptible animals in which infection rates of 100 and 95 per cent were achieved by the inoculation of 50 sporocysts. By the application of antilymphocytic serum, cyclophosphamide, irradiation or corticosteroids infection rates of 83, 92, 98 and 100 per cent, respectively, could be achieved in NMRI-mice. For future chemotherapeutical trials an inoculation dose of 50 sporocysts into irradiated NMRI-mice is recommended. It is suggested that the model is also suitable for the screening of drugs for their efficacy against exoerythrocytic schizonts of Plasmodium.