The coupling of the quinoprotein glucose dehydrogenase to the electron transport chain has been investigated in . No evidence was obtained to support a previous suggestion that the soluble form of the dehydrogenase and the soluble cytochrome associated with it are involved in the oxidation of glucose. Analysis of cytochrome content, and of reduction of cytochromes in membranes by substrates, and of sensitivity to cyanide indicated that glucose, succinate and NADH are all oxidized by way of the same -type cytochrome(s) and cytochrome oxidases (cytochrome and cytochrome ). Mixed inhibition studies [with KCN and hydroxyquinoline -oxide (HQNO)] showed that the -type cytochrome(s) formed a binary complex with the -type oxidase and that there was thus no communication between the electron transport chains at the cytochrome level. Measurements of the reduction of ubiquinone-9 by glucose and NADH, and inhibitor studies using HQNO, indicated that the ubiquinone mediates electron transport from both the glucose and NADH dehydrogenases. In some conditions the quinone pool facilitated communication between the ‘glucose oxidase’ and ‘NADH oxidase’ electron transport chains, but in normal conditions these chains were kinetically distinct.