The influence of MNDP on respiratory rate, oxidative phosphorylation and redox equilibrium between respiratory chain cytochromes (cyt) and mitochondrial membrane cyt b5 was studied by polarography and transmittance differential spectroscopy (BRAUSER 1967). O2 consumption is enhanced 18 μAtO/min.mg protein in the substrate free state 2 and controlled state 4. K1/2 =7.4×10−5M. The increased O2consumption is not inhibited by amytal, malonate, antimycin A; it is inhibited by cyanide. The respiratory rate of isolated microsomes is uneffected by MNDP. These data suggest an electron flow from MNDP to the cyt c-aa3 region of the respiratory chain. this is confirmed by the lack of oxidative phosphorylation during MNDP oxidation (no influence of ADP, DNP, oligomycin). Spectroscopically an electron flow from mitochondrial membrane cyt b5 to cyt aa3 is established, mediated by MNDP. This membrane related redox system may be of clinical relevance: hereditary cyt b5 reductase deficiency is accompanied by severe mental retardation.