The antitumor and antispermatogenic agent lonidamine inhibits Trypanosoma cruzi epimastigotes growth in culture with an ID50 around 80 microM. The main site of action appears to be the mitochondria, where the rate of uncoupled respiration was inhibited in 50% at a similar lonidamine concentration (50 microM). Hexokinase (the other point where lonidamine inhibits tumor energy metabolism) was not sensitive to this drug. Lonidamine also inhibited uncoupled respiration in T. brucei procyclic trypomastigotes, suggesting a common mechanism of action with T. cruzi. When lonidamine was added to T. brucei trypomastigotes, there was little effect on the CN-insensitive respiration, demonstrating that at least in T. brucei glycolysis is not affected by the drug.