The electrocatalytic reduction and oxidation of nitric oxide on electrodes modified with Prussian blue (PB) films in acidic aqueous buffer solutions has been studied. The cyclic voltammetry and spectroelectrochemistry results show that Prussian white, the reduced form of Prussian blue, acted as an electrocatalyst toward NO reduction. The reduction current started at - 0.4 V and became higher at more negative potentials. Bulk electrolysis of nitrite solution at PB-modified electrodes at - 0.7 V led to the formation of NH3 and NH2OH. Berlin green, the oxidized form of PB, has the activity to catalyze NO oxidation to yield nitrate at + 1. 3 V. The reactivity of PB is discussed and the reaction schemes are presented.