A polypyrrole-containing Fe(CN)(6)(3-) modified electrode was prepared by anodic electropolymerization at 0.8 V versus SCE of an aqueous solution containing only pyrrole and K4Fe(CN)(6). The concentration of electroactive Fe(CN)(6)(3-) ions in the polymer was found to be 30 times higher than that of the ferrocyanide ions in the electrolytic solution. Furthermore the Fe(CN)(6)(3-)/Fe(CN)(6)(4-) redox system exhibited a high degree of reversibility. These properties made it possible to use the modified electrode as a pseudo-reference in a weakly polarized two-electrode device for the design of amperometric biosensors involving NAD-dependent dehydrogenases. The assay of D-lactic acid was taken as an example using D-lactate dehydrogenase and diaphorase. The sensitivity of the biosensor, i.e. 20 muA mM(-1) cm(-2), was similar to that in previous studies. The modified electrode exhibited a relatively stable potential for currents lower than 100 nA and had an operating life of more than 2 months. (C) 2000 Elsevier Science Ltd. All rights reserved.