In guinea pigs with acute heart failure, a decrease in oxygen consumption (QO2) at state 3 and an increase in QO2 at state 4 were seen and the respiratory control index (RCI) was markedly lowered; ADP/O ratio was not affected. In rabbits with acute heart failure, both RCI and ADP/O ratio showed a statistically significant decrease. In dogs with acute and chronic heart failure, no impairment of respiratory function of mitochondria was seen. These conflicting results may be attributed to the difference in experimental conditions, namely, the species difference and the difference in methods to produce experimental heart failure. With special regard to species difference, it was confirmed that the RCI of myocardial mitochondria of healthy dogs was much lower than that of healthy guinea pigs. It is suggested that the difficulty of obtaining intact mitochondria may be responsible for the species difference in experimentally induced heart failure.