Mexiletine, 1-(2,6-dimethylphenoxy)-2-aminopropane (Mexitil), is an orally effective agent useful in the treatment of serious ventricular arrhythmias. This paper describes a gas chromatographic-mass spectrophotometric assay with selected-ion monitoring for the measurement of plasma or serum mexiletine levels. The drug and internal standard (p-chlorophenylalanine methyl ester) were extracted from plasma into ethyl acetate-hexane-methanol (60:40:1, v/v). After separation and evaporation of the organic phase, the drug and internal standard were derivatized to their pentafluoropropyl derivatives prior to analysis. The reproducibility of the daily standard curve yielded mean inter- and intra-day coefficients of variability from 0.7 to 11.0%. The coefficients of variability for control plasma samples (0.5 and 1.0 micrograms/ml) ranged from 2.6 to 5.0% and the accuracy of the assay was 106 +/- 6 and 100 +/- 5% for the low and high level controls respectively. The limit of quantitation for the assay was 0.1 micrograms/ml. No interfering peaks were detected in any patient samples. This method can be used as a primary analytical method to measure mexiletine plasma levels or can serve as a convenient back-up method to HPLC procedures when contaminating peaks coelute with mexiletine.