Summary 1. The dog, harboring Dirofilaria immitis, serves as an appropriate laboratory animal for testing the filaricidal effect of drugs proposed for use in the treatment of human filariasis. 2. Fatal doses of fouadin given intramuscularly, within a short period of time caused a complete disappearance of the microfilariae from the peripheral blood of the dog, but exerted little or no damaging effect on the adult worms in the heart or pulmonary artery. 3. Carbarsone, hexylresorcinol, trypan blue or sodium-iodoantimonite did not possess any marked filaricidal or microfilaricidal qualities when tested on dogs infested with the heart-worm.