A poly(Toluidine Blue O) (poly-TBO) modified electrode was successfully prepared by repeated sweeping the applied potential from -0.6 to + 0.8 V (vs. SCE) on a glassy carbon electrode (GCE) in borate buffer solution at pH 9.1 containing 0.1 M NaNO3 and 0.4 mM Toluidine Blue 0 (TBO). The poly-TBO modified GCE shows electrocatalytic activity toward NADH oxidation in phosphate buffer solution at pH 7.0, with an overpotential of ca. 350 mV lower than that at the bare electrode. The photoelectrocatalytic oxidation of NADH at this electrode was also successfully investigated by using cyclic voltammetry and amperometry at constant potential. When the modified electrode surface was irradiated with a 250 W halogen lamp, a photoelectrocatalytic effect was observed for NADH oxidation and the current was increased about 2.2 times. The applied potential was selected at +100mV for amperometric and photoamperometric detection of NADH. A linear calibration graph for NADH was obtained in the range between 1.0 x 10(-5) and 1.0 x 10(-3) M and between 5.0 x 10(-6) and 1.0 X 10(-3) M for amperometric and photoamperometric studies, respectively. The effect of some interfering compounds, such as ascorbic acid and dopamine on the electrocatalytic and photoclectrocatalytic oxidation of NADH was tested.