Methylcyclopentadienyl manganease tricarbonyl (MMT) has been developed and used increasingly as a replacement for tetraethyl lead which has been widely used as an antiknock agent of gasoline, or as a fuel oil combustion improver. We, therefore, investigated the effect of MMT on oxidative phosphorylation and K release of mitochondria as an index of toxic effects on biomembranes. The concentrations of solution used were 0.10, 0.25, 0.40 and 0.50 mM. Results obtained were as follows. 1. MMT activated state 4 respiration at more than 0.20 mM solution, i.e. showing the uncoupling phenomenon. 2. Mitochondrial oxidative phosphorylation with MMT was greater than with MnCl. 3. Both MMT and MnCl(2) induced K release from mitochondria, and a dose-response relationship between K release and MMT was recognized. 4. It was considered that one reason for the remarkable difference in uncoupling between MMT and MnCl(2) was the difference in lipid affinity.