The effect of mono-, di-, tri- and tetra-chlorinated benzene on K(+)release from rat liver mitochondria was determined. The following results were obtained: 1) There were differences between the isomer of dichlorobenzenes on the K(+)release and increase in the K(+)release was in the order of ortho > meta > para dichlorobenzenes. 2) K(+)release increased with increase in the number of chlorine. 3) K(+)release increased with increase in the concentration of chlorinated benzene.