The electropolymerization of a flavin reductase-amphiphilic pyrrole ammonium mixture previously adsorbed on the electrode surface provides the efficient entrapment of the enzyme in a functionalized polypyrrole film. The enzyme film was characterized and applied to the detection of several flavins and pyridine nucleotides. The response of the bioelectrode was based on the amperometric oxidation, at -0.1 V vs SCE, of the enzymically generated dihydroflavins in the presence of reduced pyridine nucleotides. The highest sensitivities and detection limits were 668 mA M(-1) cm(-2) and 4 nM for riboflavin, 179 mA M(-1) cm(-2) and 20 nM for flavin mononucleotide, and 107 mA M(-1) cm(-2) and 40 nM for flavin adenine dinucleotide. Direct electrochemical measurements at -0.65 V and electroenzymatic measurements at -0.1 V of riboflavin, carried out with these bioelectrodes, show that an amplification phenomenon occurs, due to an electrochemical recycling reaction of enzyme substrate. A nonamplified response to reduced pyridine nucleotides was also obtained in the presence of 0.1 mM riboflavin. The sensitivity and detection limit of the bioelectrode are 19.6 mA M(-1) cm(-2) and 0.2 μM for NADH and 11.2 mA M(-1) cm(-2) and 0.5 μM for NADPH.