The antimalarial properties of crude extracts of plants belonging to the genera Dichroa and Hydrangea have been described in previous papers in this series (Duggar, in press;Ablondi et at. 1952; Hutchings et at. 1952). After the activeprinciple was isolated in crystalline form from the leavesof a varietyof Hydrangea sp. (Ablondi et at. 1952), degradation (Hutchings et at. 1952) and synthesis (Baker et at. 1952) proved it to be an alkaloid, 3-[@-keto-7-(3-hydroxy-2-' piperidyl)propyl]-4-quinazolone. The naturallyoccurringalkaloidwas found to have a quinine equivalent of approximately 100 when tested in ducks infected with Plasmodium lophurae, and the dl form of the alkaloid prepared synthetically was about 3@as effective. Previous studies of alkaloids isolated from DichroG febrifuga, and tested against P. gallinaceum in chickens (Coatney et at. 1950), P. lophurae in ducks (Henderson et at. 1949), P. relictum in canaries (idem), and P. cynomolgi in monkeys (idem), have given quinine equivalents of from 64 to 148. Among the names given to the naturally occurring alkaloids of Dichroa febrifuga have been Dichroine (Chou et at. 1947), y-Dichroine (Henderson et at. 1949; Chou et at. 1948), and Febrifugine (Henderson et at. 1949). Febrifugine has been reported to be active against the Chesson strain of P. vivax in humans (Coatney etat.1950).It has been demonstratedthat Febrifugine and thenat urally occurring antimalarial alkaloid isolated from Hydrangea sp. in these laboratories areidentical (Ablondietat.1952).Since the alkaloid from Hydrangea possessed a high quinine equivalent, and a method of synthesis was accomplished, it was considered desirable to investi gate related quinazolones for antimalarial activity. The results of assays of 54 synthetic quinazolones against P. lophurae in ducks are described herein, and further data will be presented as the series is expanded. METHODSWhite Pekin ducklings, 2 weeks old and about 200 grams in weight were used as experimental hosts. Infections with P. lophurae were produced by injecting intravenously 0.5 cc. of citrated blood from a heavily parasitized donor bird. The peak ofparasitemia was reachedon the 5th day in control birds, and blood smears were taken at that time to determine the effects of treatment. All of the compounds listed in the tables were soluble in water, and were usually supplied in the form of the hydrochloride salt. They were administered orally by tube twice daily for 3@ days. One dose was given on the day of inoculation, with two doses daily thereafter for 3 days. The dosage in milligrams per kilogram 768