2.3 micromoles/mg protein of MFNI induced a 60% decrease in the heart mitochondrial ADP-stimulated oxygen uptake using glutamate-malate as substrate. The same amount of niridazole, ipronidazole, DA 3851 and ornidazole led to falls of less than 20% in the oxygen uptake, whilst metronidazole was ineffective. State 3 and state 3 mu (uncoupled) respiration were affected to the same extent. Oxygen-uptake using succinate as substrate was not inhibited indicating that the action was exerted at the NADH oxidation level. The relationship between electroreduction potentials of the test compounds and inhibition of respiration has been studied.