Chemically, pyrimethamine (Daraprim) is 2,4-diamino-5-p-chlorophenyl-6-ethylpyrimidine, with the empirical formula C12H13N4Cl. This chemical was synthesized by the Wellcome Research Laboratories, Tuckahoe, N. Y., as an antimalarial drug which arrests plasmodial development at those stages involving nuclear division. Besides this antimalarial activity, pyrimethamine has been shown to have an extraordinarily potent antimetabolite activity. To my limited knowledge, the mode of action of the preparation against toxoplasmosis has not been entirely elucidated. However, Eyles and his associate,1 in 1952, reported antitoxoplasminic action of the drug in mice and, in 1953, showed that pyrimethamine and sulfadiazine were synergistic to each other against experimental toxoplasmosis in the mouse.2 Since these reports, the drug has been widely used in the treatment of presumptive toxoplasmic infections of man as well as of experimental toxoplasmosis of animals. In the past years, numerous papers on the subject have been published, creating