A new method for determining pilsicainide concentration in serum by rapid and selective capillary electrophoresis has been developed and validated. For pretreatment, serum was made alkaline and then extracted with diethyl ether. Procainamide was used as an internal standard. Sodium dihydrogenphosphate buffer (pH 2.29; 0.1 M) was used as a running buffer. A fused-silica capillary tube was loaded with a voltage of 25 kV and detection was performed at UV 200 nm. Good linearity (0-2.5 microg/ml) was obtained with the minimum limit of detection being 0.04 microg/ml serum (signal-to-noise ratio, 3:1). The R.S.D. of within-run reproducibility was 0.798-2.32%, that of between-run reproducibility was 4.74-5.12% and the recovery rate was 61-63%. Disopyramide, another anti-arrhythmic drug, was close to pilsicainide in terms of migration time. This method was applied to determination of pilsicainide in serum samples.