Abstract: The development of N. braziliensis was studied both in rats and mice.From the observations made on the development rate of adult worms,the worms' distribution in the small intestine, and the number of worms passing out spontaneously, it was concluded that rats are better laboratory animals for our purpose than mice. A routine technique for screening anthelmintics against hookworm in vivo was estab- lished. Young laboratory rats (90-110 g in wt.) are each inoculated subcutaneously with 400 active N. braziliensis infective larvae, and after 11 or 12 days, drugs to be tested are given per os. A group of 4 rats are used for each drug,and no control group is neccssaey. Faecal material is collected for 48 hours and the rats are then sacrificed, worms being searched carefully both in the intestines and in the faeces. The total number of worms in the intestine and faeces of the whole group of animals are counted, and the ratio is termed the direct therapeutic index: Among the ten known antihookworm drugs and broad-spectrum anthelmintics tested, eight were highly effective, the therapeutic index being 69.5-100%. Oil of chino- podium and stilbazium iodide are the only exceptions. Regarding the drugs known to be ineffective against hookworms, the therapeutic index was almost nil. The results appear to prove that the new method is not only time saving and economical but also more ac- curate as compared with the methods reported previously.