A comparative study of the effects of tetrachlorobiphenyls (TCBs) on the succinate-supported respirations of rat liver mitochondria was made, and some differences in effects caused by the different chlorine positions of the biphenyl rings were clarified. The inhibitory actions of 2, 3, 2', 3'-, 2, 4, 2', 4'-, and 2, 5, 2', 5'-TCBs on both state 3 and uncouplerstimulated respirations were potent, while those induced by 2, 6, 2', 6'-, and 3, 4, 3', 4'-TCBs were weak. 2, 3, 2', 3'-, 2, 4, 2', 4'-, 2, 5, 2', 5'-, and 2, 6, 2', 6'-TCBs stimulated state 4 respiration, but 3, 4, 3', 4'-TCB had very little effect on this respiration. The latent adenosine triphosphatase activity was stimulated by 2, 3, 2', 3'-, 2, 4, 2', 4'-, and 2, 5, 2', 5'-TCBs, but 2, 6, 2', 6'-, and 3, 4, 3', 4'-TCBs had no effects. The relationship between these effects and chemical structure of TCBs is discussed.