Effect of PCNs on normal isolated mitochondrial membrane was studied and the results obtained were as follows. 1) PCNs inhibited the state 3 respiration, and stimulated the state 4 respiration (namely uncoupling action). 2) The proportional relationship between a decrease in RCI and an increase in concentration of Halowax 1001 (PCN) was recognized, and a decrease tendency of ADP/O ratio was observed within the range of PCN concentration measured. 3) PCNs induced a K(+) release in mitochondria and the release effect was the strongest in Halowax 1031 (PCN). 4) Discussion was made that effects of PCNs on the mitochondrial membrane were seemed to be related to their complex characters which are the content of chlorine and their isomers.