A high performance liquid chromatographic determination of antiarrhythmic agents: propafenone (PPF) with its principal active metabolite 5-hydroxypropafenone (5-OHPPF), amiodarone (AD) with an active metabolite desethylamiodarone (DEAD), mexiletine (MEX) and diltiazem (DILT) in human serum is presented. The compounds are separated on Supelcosil LC-CN column at ambient temperature under isocratic conditions. For PPF, 5-OHPPF, MEX and DILT analysis the mobile phase: CH3CN: H2O: 0.5 M KH2PO4(360:620:20 v/v) is pumped at a flow rate of 1.8 mL/min and the ultraviolet detection is performed at 210 nm. AD and DEAD are eluted with stronger mobile phase: CH3OH: CH3CN: H2O: 0.5 M KH2PO4(480:136:360:24 v/v) at a flow rate of 1.5 mL/min and detected at 240 nm. Sample preparation is based on simple liquid-liquid extraction from acidified serum for AD and DEAD and on liquid-liquid extraction from alkalized serum followed by back extraction for PPF, 5-OHPPF, MEX and DILT analysis. Sensitivity of the assay was set at 5 ng/mL for PPF and MEX and at 10 ng/mL for 5-OHPPF, DILT, AD and DEAD. Coefficient of variation for intra- and interassay precision was below 10% for all the tested compounds in all the tested concentrations. The method is convenient for therapeutic monitoring of those four antiarrhythmics. Described assay may be easily adapted for the determination of verapamil, gallopamil, quinidine, flecainide and a few beta-blockers.