Lidocaine is widely used in antiarrhythmic therapy, and the need to monitor the drug concentration in the blood of patients receiving lidocaine is well established. We have developed a new assay for lidocaine designed for the routine clinical therapeutic drug monitoring laboratory. A 100-μl aliquot of blood plasma is treated with 50 μl of internal standard solution and the mixture alkalinized. The drug and internal standard are extracted with toluene and analyzed by nitrogen-detection gas-liquid chromatography. A single extraction is used, and no solvent evaporation is required, resulting in a simple and rapid procedure. Using the single-point standard, the within-day coefficient of variation (CV) was 6.4%, while the between-day CV was 7.4%. Monoethylgly-cinexylidide, the major plasma metabolite of lidocaine, and several drugs potentially present in patients receiving lidocaine were shown not to interfere. The procedure has significant advantages over previously published similar methods.