Bretylium [(o-bromobenzyl)ethyldimethylamine] is a quaternary ammonium compound used as the tosylate salt for treatment of ventricular fibrillation in humans. A sensitive assay was developed for the determination of low bretylium concentrations in plasma and urine. The internal standards were (p-bromobenzyl)ethyldimethylammonium p-toluenesulfonate and (o-methoxybenzyl)ethyldimethylammonium p-toluenesulfonate. Samples were deproteinized with acetonitrile and extracted with methylene chloride. After the evaporation of the organic phase, the residue was reacted with sodium 2,4,5-trichlorothiophenolate in methanol. This procedure yielded volatile compounds with excellent electron-capture capabilities for the GLC analysis. The assay sensitivity is 5 ng/ml. The extraction recovery of bretylium as determined by a direct radioactivity measurement was 90 and 97% for plasma and urine, respectively. The method is highly reproducible with no significant day-to-day variations. Comparisons of 60 standard plasma samples, 25 standard urine samples, and plasma samples from a dog that received [14C]bretylium showed excellent agreement between the GLC method and direct radioactivity measurement of bretylium.