An inexpensive and simple nitric oxide biosensor is developed with a peroxidase amperometric electrode using hydrogen peroxide as substrate and p-benzoquinone as mediator. The enzyme has been immobilized on a glassy carbon electrode or on a nylon mesh which is attached to the electrode. The influence of the immobilization method on the biosensor response has been studied. Peroxidase reduces hydrogen peroxide to water and oxidizes the mediator which is regenerated in a cathodic reaction. Cyclic voltammetry has been employed to assess the ability of certain redox-active couples to act as mediator. The activity of horseradish peroxidase is inhibited in the presence of nitric oxide. Thus, the decrease in activity of the enzyme monitored can be correlated to the concentration of nitric oxide present in solution. This biosensor responds to 2.7 × 10−6−1.1 × 10−5 M nitric oxide. A detection limit of 2.0 × 10−6 M for nitric oxide was found.