The behavior of meso-tetraphenylporphyrin (H2TPP) and its Co(III)-complex (CoTPPCl) in Nafion((R)) film has been studied by cyclic voltammetry and visible spectroscopy. Buffer titration of H4TPP2+ in the film showed only the dication-free-base equilibrium (H4TPP2+ reversible arrow H2PP + 2H(+)) and the apparent equilibrium constant, pK(3.4), was determined to be 4.12. Insertion of cobalt(III) into the H2TPP core in the film has been realized at room temperature by immersion of H2TPP-containing Nafion((R)) film into CoCl2 solution. The electrocatalytic activity of the resulting film toward NO oxidation at a glassy carbon (GC) electrode has been monitored in comparison with other Nafion((R)) films doped with H2TPP or free CoCl2 A GC electrode coated with Nafion((R)) film doped with CoTPPCl exhibited the highest catalytic activity, confirming the involvement of the Co site in a catalytic cycle of NO oxidation. In amperometric detection of NO in phosphate buffer solution at pH 7.4 this electrode demonstrated a linear response to the NO concentration increment. The detection limit at a signal-to-noise ratio of 3 was 1.2 nM.