Ethanol extract from Arrabidaea triplinervia leaves showed in vitro activity (ED100 5.0 mg/ml) against trypomastigotes of Trypanosoma cruzi, the etiologic agent of Chagas; disease. Bioactivity-directed fractionation of this extract led to the isolation of ursolic and oleanolic acids as trypanocidal compounds besides pomolic acid (not tested) and alpinetine (inactive). A series of natural and synthetic derivatives of ursolic and oleanolic acids was simultaneously assayed for structure activity relationships (SAR) studies. Ursolic acid (ED100 0.4 mg/ml) was four times more active than oleanolic acid (ED100 1.6 mg/ml). The presence of free hydroxy and/or carboxy groups is necessary for the trypanocidal activity as could be deduced from the effect of the acetates, methyl ester, and aldehyde derivatives.