A rapid, reliable assay is described for the quantitation of disopyramide (DP) and its metabolite, mono-N-desisopropyl-disopyramide (MNDP), in serum using p-chloro-disopyramide as the internal standard and aqueous standards for calibration. The procedure involves extraction of the serum sample with diethyl ether, derivatization of MNDP with acetic anhydride, and gas-liquid chromatographic analysis with nitrogen-selective detection. The accuracy of the procedure is 97.8 +/- 10.3 (SD)% for DP over the concentration range, 0.25-10.00 micrograms/ml (n = 37), and 99.6 +/- 10.2 (SD)% for MNDP over the concentration range, 0.50-5.00 micrograms/ml (n = 18). The within-day coefficient of variation is less than 8% for DP and 12% for MNDP. The lower limit of quantitative sensitivity is 0.25 micrograms/ml for DP and 0.50 micrograms/ml for MNDP, and the lower limit of detection is 0.6 ng and 1.6 ng, respectively, for DP and MNDP. This assay has been used to determine the time-course of DP and MNDP and the protein binding of DP (without requiring an in vitro protein binding curve) in the serum obtained from patients and laboratory animals during DP administration.