The NADPH oxidizing activity of rat liver microsomes was investigated and found to be mainly due to the cytochrome P-450 system. The XADPH oxidase was utilized for the development of several organelle electrodes. Gelatin membrane immobilized microsomes were combined with an O2membrane sensor for bioelectrochemical measurement of NADPH. The dependence of the current on substrate concentration was linear up to 1 mmol·1−1To assemble hybrid electrodes for determination of glucose-6-phoaphate, ATP and isocitrate pure enzymes were coimmobilized with the microsomal fraction.