An NO sensor based on a carbon fibre microelectrode modified by a poly(N-methylpyrrole) incorporating [(H2O)(FePW11O39)-P-III](4-) sublayer and coated by a Nafion(R) external layer is described. It is based on the NO oxidation current measured by differential normal pulse voltammetry. In solution, a linear response is obtained between this oxidation current and the NO concentration in the range 10(-7) to 10(-3) M with a sensitivity of 2.65 +/- 0.15 nA mu M-1. A comparative study has been carried out with poly(N-methylpyrrole) doped with ClO4-, polypyrrole doped with sulfonated nickel porphyrin, polypyrrole and poly(N-methylpyrrole) doped with Nafion(R). This sensor implanted in the rat brain is efficient to detect in real time the NO released, The validation of in vivo measurements is made by injecting the rat with an NO-synthase inhibitor.