New PVC membrane electrodes for the determination of sulfadiazine (SDZ) are presented. The electrodes are fabricated with conventional and tubular configurations with a graphite-based electrical contact, and no internal reference solution. The selective membranes consist of bis(triphenylphosphoranilidene)ammonium.SDZ (electrode A), tetraoctylammonium bromide (electrode B), or iron(II)-phthalocyanine (FePC) (electrode C) electroactive materials dispersed in a PVC matrix of o-nitrophenyl octyl ether (o-NPOE) plasticizer. The sensors A, B, and C displayed linear responses over the concentration ranges 1.0 x 10(-2) - 1.0 x 10(-5), 1.0 x 10(-2) - 7.5 x 10(-6), and 3.2 x 10(-2) - 7.0 x 10(-6) mol l(-1) (detection limits of 1.09, 2.04 and 0.87 microg ml(-1)) with anionic slopes of -57.3 +/- 0.1, -46.7 +/- 0.5, and -65.1 +/- 0.2 mV decade(-1), respectively. No effect from pH was observed within 4.0 - 5.5, 4.8 - 10, and 4.5 - 8, respectively, and good selectivity was found. The sensors were applied to the analysis of pharmaceuticals and biological fluids in steady state and in flow conditions.