ABSTRACT
Plasmodium yoelii yoelii-Anopheles stephensi system was chosen as the experimental model in studying the effects of pyrimethamine on oocyst formation of the plasmodium species. The drug was given by allowing mosquitoes to feed on infected and pyrimethamine treated mice or by feeding them directly with pyrimcthamine-sugar water. The infective rate and the number of oocysts formed after drug administration were reduced, the oocysts formed being smaller and their daily growth rate slower than that of the controls. Electron microscopic and Feulgen staining studies showed that the cytoplasm of the affected oocysts contained many vacuoles, pigment aggregations and black aggre-gates (Fig. 4). No nucleus appeared in the affected oocysts, which were presumably deteriorated and became "black spores". The amount of DNA in drug-affected oocysts was scanty. No sporozoites were found in the salivary glands of these mosquitoes. It was suggested that pyrimethamine interfered with DNA synthesis of oocysts.