The mitochondrial oxidative phosphorylation system was studied in liver and heart homogenates after treatment of rats with benznidazole. The drug was given by oral gavage to adult female Wistar rats for 9 consecutive days (100 mg benznidazole/kg body weight as a daily dose). The mitochondrial state 4 and state 3 respiration rates, respiratory control, efficiency of oxidative phosphorylation (ADP/O), and ATPsynthase activity were assayed. The results showed that according to all these parameters, the mitochondria in cardiac homogenates were not affected in the rats treated with benznidazole. By contrast, mitochondria in the liver homogenates of drug-treated rats were altered, showing decreased respiratory control and a lower coefficient of ADP/O as a result of an increase in the state 4 respiration rate. These data indicate the possibility of production of an uncoupling factor leading to increased proton leakage through the inner mitochondrial membrane as a result of a 9-day treatment of rats with benzonidazole. The obtained experimental data might at least partly explain the nature of benznidazole toxicity in the liver treated with benznidazole.