A comparative study was made of the effects of biphenyl and polychlorinated biphenyls (Kanechlors®, KCs) on the respiratory and energylinked activities of rat liver mitochondria. With αketoglutarate/malate as the substrate, biphenyl acted as the strongest inhibitor of state 3 respiration. The inhibition of state 3 respiration was decreased as chlorine content increased. The stimulation of state 4 respiration was greatest in KC-200, 300, and 400; intermediate in biphenyl and KC-500; least in KC-600. When succinate was the substrate, from the results of this and a previous study (Nishihara 1983) the action of biphenyl as an inhibitor of state 3 respiration was least effective. The inhibitory action on state 3 respiration was strengthened by the chlorination of aromatic rings (KC-200), reaching a peak where maximum inhibition was observed (KC-300, 400, and 500), further increases in chlorine content (KC-600) repressed the inhibitory action. The stimulating ability of state 4 respiration was greatly diminished with increases in chlorination. Biphenyl and KCs induced the K+ -release from mitochondria, thereby dissipating membrane potential across the mitochondrial membranes in the order; biphenyl > KC-200 > KC-300 > KC-400 > KC-500 > KC-600. It is concluded that with α-ketoglutarate/malate (NAD+ -linked substrate), KCs act as uncouplers by increasing the permeability of mitochondrial inner membranes to ions, and that when succinate is the substrate, KCs act as inhibitors rather than uncouplers of oxidative phosphorylation of mitochondria by inhibiting the electron transport chain.