The aerobic oxidation of reduced pyridine phosphonucleotides catalyzed by horseradish peroxidase (E.C.1.11.1.7) (HRP) in combination with Mn2+-ions, phenolics and redox dyes was used to perform NADH and NADPH determinations with an amperometric HRP enzyme electrode. Optimal operational conditions have been elaborated for such a sensor and a linear relationship between derivative current (dI/dt) and NADH-concentration was obtained between 2.10−5and at least 2.10−4mol/l. The coefficient of variation was equal to or smaller than 4 %. The time for one measurement was less than 10 minutes including equilibration of the electrode.