Microbial nitrate reductase from Pseudomonas stutzeri (NaR, EC 1.7.99.4) was tested in amperometric enzyme electrodes for detection of nitrate. The membrane-bound enzyme was coadsorbed with 19 different mediators onto graphite. We used the resulting sensors to characterize the bioelectrochemical properties of mediators of phenothiazine, triphenylmethane, sulfon-phthaleine and viologen types, and studied the dependence of the current densities on the electrode potential and nitrate concentration under anaerobic conditions. Redox mediators belonging to the phenothiazine (Azure A, Thionin) and to the triphenylmethane dye group (Patent blue) performed best. Azure A gave the highest current density of 70 mA cm(-2) under saturating nitrate concentration. The response time of the sensors was 12 s, the nitrate detection limit was 4.8 mmol l(-1) and the half lives of the sensors were of the order of one to two days.