Ion-selective poly(vinyl chloride) membrane electrodes and thick-film sensors for the determination of the antiarrhythmic drug bretylium in human serum are described. Ion-pair complexes of bretylium with the anionic counter ions tungstosilicate, reineckate [diamminetetracyanatochromate(III)], tetraphenylborate and dipicrylaminate were investigated as electroactive compounds for the electrode membranes. The membranes containing the bretylium–tungstosilicate complex showed the best properties with both types of transducers. The detection limits were determined to be 1 × 10^–7 mol dm^–3 for the macroscopic electrodes and 8 × 10^–7 mol dm^–3 for the thick-film sensors in 0.1 mol dm^–3 Tris buffer at pH 7.4; slopes of 60.7 and 40.0 mV decade^–1 respectively, were found. In pathological and non-pathological test sera, a detection limit of 5 × 10^–6 mol dm^–3 was obtained with the thick-film transducers. The potentiometric selectivity coefficients towards structurally similar compounds such as ephedrine, dopamine and epinephrine (adrenaline) ranged from 10^–1.7 to 10^–3.4. Response times of 5–10 s were observed in buffered aqueous solutions as well as in human serum samples.