For all members of the chlorinated ethane and ethylene series except monochloroethylene, the effects on the oxygen consumption of isolated rat mitochondria were studied. All the tested compounds inhibited glutamate and malate oxidation, and their inhibitory potency increased in the order of the number of contained chlorines. The eight compounds of greatest potency were also investigated for their inhibition of succinate oxidation in mitochondria and inhibition of NADH oxidation in sonicated submitochondrial particles. The data obtained suggested that the inhibition of substrate oxidation was due to blockade of electron transport and the most susceptible portion was located from NADH to CoQ.