This communication reports on the first electrochemical study of the human flavin-containing monooxygenase 3 (hFMO3) either absorbed or covalently linked to different electrode surfaces. Glassy carbon and gold electrodes gave reversible electrochemical signals of an active hFMO3. The midpoint potential measured for the immobilized enzyme on a glassy carbon electrode was -445 +/- 8 mV (versus Ag/AgCl). A monolayer coverage was obtained on gold functionalized with dithio-bismaleimidoethane that covalently linked surface accessible cysteines of hFMO3. A structural model of the enzyme was generated to rationalize electrochemistry results. The turnover of the active enzyme was measured with two specific drugs: tamoxifen and benzydamine. For tamoxifen, 1.7 and 8.0 microM of its N-oxide product were formed by the enzyme immobilized on glassy carbon and gold electrodes, respectively. In the case of benzydamine, a K(M) of 44 +/- 5 microM was measured upon application of a -600 mV bias to the enzyme immobilized on the glassy carbon electrode that is in good agreement with the values published for microsomal hFMO3 where NADPH is the electron donor.