Rapid analyses of drug metabolism reactions by human cytochrome P450s (CYPs) that metabolize 95% of all current drugs are very important in drug development processes and effective therapies. Since CYPs need electrons to metabolize drugs, electrons supplied from electrodes to activate CYP molecules are expected to be very useful to develop rapid assay methods for CYP reactions. Although several studies on the direct electrochemistry of isolated and purified human CYPs have been reported, the use of microsomes (membranes) containing CYP is more suitable, because they are frequently used in drug research due to their easy preparation and low cost. Herein, we demonstrate electrons supplied from an electrode to microsomes containing CYP and CYP-reductase (CPR) on an electrode coated with hydrophobic thin films and observe electrochemically driven drug metabolisms by voltammetry for the first time. We tested the immobilization of microsomes on gold electrodes coated with several thiolates and found that microsomes immobilized on thin hydrophobic surfaces of aromatic compounds showed welt-defined redox peaks. Furthermore, electrochemically driven drug metabolism reactions of CYP3A4 were clearly observed by voltammetry, and these reactions were inhibited by a CYP3A4 inhibitor, ketoconazole. We also found that metabolism reactions were facilitated in the presence of CPR.