The cinchona alkaloids, or those derived from them, were the only ones tested that showed antimalarial activity against Plasmodium inconstans. Hydroqui-nine was the most active, followed by quinidine, quinine, cinchonidine and cinchonine in the order named. Hydrogenation of quinidine, cinchonidine and cinchonine did not lead to greater activity. The activity was restored to quitenine and the analogous carboxylic acids derived from quinidine, cinchonidine and cinchonine by esterification. The activity increased as the carbon atoms of the alcohol increased, the maximum being at butyl- or amyl-quitenine.