Abstract: For the purpose of screening new oral filaricides, cotton rat filaria model consisting of cotton rats (Sigrnodon hispidus) infected with Litomosoides carinii, and rat mites (Ornithonyssus bacoti) were introduced in 1972 through the courtesy of Dr. E. Friedheim. Since then, the model has been well mainteined in our Institute and fully utilized by other institutions.During the past five years, 150 known or newly synthesized compounds and 7 medicinal herbs had been screened. 14 compounds showed obvious microfilaricidal and/or macrofilaricidal effects, among them 4-isothiocyanato-4'-nitrodiphenylamine, 2, 4-diamino-6-(3,4-dichloro-benzyl) nitrosamino quinazoline acetate, Furacilin, 2-(4-melamin-2-ylphenyl)-4-hydroxymethyl-1, 3-dithia-arsolane (syn, Melarsoprol, Arsobal, Mel B), 6-P-nitro-phenyl-2, 3, 5, 6-tetrahydroimidazol-(2, 1-b) thiazole and five derivatives of 2-2-(5-nitrofuryl-2) vinyl-1-1, 3, 4-oxadiazole-5-one were more outstanding, especially as macrofilaricides.The experimental methods pertinent to the drug screening and the variation of microfilaria and adult worm counts in cotton rats are described.