Gas-liquid chromatography using capillary columns and electron-capture detection has been employed for the determination of a new antiarrhythmic drug, tocainide, in rat plasma and urine. The drug is extracted from a basified solution along with an internal standard, monoethylglycine xylidide, and subsequently reacted with heptafluorobutyric anhydride. The 50 m x 0.2 mm fused silica capillary column was coated in the laboratory with Carbowax 20M. Linearity of detector response was established in the range of 50-1000 ng of tocainide hydrochloride per 100 microliter of plasma or urine. This represented 14-270 pg of the free base at the detector, using a split ratio of 1:25 and an injection volume of 2 microliter. The derivatization method and chromatographic assay are well suited for monitoring of plasma or urine samples. The applicability of the method is demonstrated by the analysis of rat plasma collected over a period of 7 h after intravenous administration of 20 mg/kg of tocainide hydrochloride.