Abstract: Feulgen stain and microspectrophotometer were used for the study of the effect of pyrimethamine on the DNAamount of oocysts of Plasmodium yoelii yoelii.The results showed that there were many DNA particles in the early stage of normal oocysts and a lot of mature sporozoits in their late stage.After pyrimethamine was used,the development of oocysts was retarded and some of them formed“black spore”.The amount of DNA in drug-affected oocysts was much lower than the control group.It was indicated that pyrimethamine interfered with DNA synthesis in oocysts,which in turn inhibited the growth of malaria parasites in mosquitoes.

Key words: Feulgen stain, Pyrimethamine, Plasmodium yoelii yoelii, oocyst