Amiodarone is an antiarrhythmic agent used for the treatment of supraventricular and ventricular arrhythmias. Owing to its narrow therapeutical range, monitoring of drug concentration is mandatory. The circulating and tissue levels of amiodarone and of its main endogenous metabolite, N-desethyl-amiodarone, are currently measured by means of HPLC procedures, which are tedious and time-consuming, and often beset with problems and drawbacks. We have developed a new chromatographic assay for the simultaneous measurement of amiodarone and N-desethyl-amiodarone in serum samples, and tested its usefulness in the follow-up of 14 patients (8 men and 6 women, age range 40-65 years) with complex ventricular arrhythmias, treated with amiodarone for at least 5 weeks. This assay uses trifluoperazine dihydrochloride as internal standard and a preliminary extraction of serum samples with isopropyl ether. The assay procedure was the following: 350 microliter patient's serum, to which 1 microgram trifluoperazine was added, were extracted with 280 microliter isopropyl ether. After mixing and centrifugation, 50 microliter of the organic layer were filtered and then injected onto the HPLC system (15 cm x 3.9 mm Resolve 5-micrometer spherical silica column); the elution rate was 1.8 ml/min (mobile phase, 920 ml of methanol and 80 ml of ammonium sulfate buffer) in isocratic condition. The time for a complete assay of each serum sample was less than 20 min, and its working range was 0.1-5.0 microgram/ml of amiodarone. An excellent recovery of the drug from subtherapeutical values up to toxic amiodarone concentration was obtained. The intra-assay precision of amiodarone assay ranged form 5 to 11%, while the between-assay precision administered in all patients, without increasing the amiodarone concentration beyond the toxic threshold level. The plateau of the circulating levels of the drug was generally reached in about 5-12 days, at the acceptable therapeutical range, from 0.5 to 2 microgram/ml. In conclusion, this chromatographic method for the assay of serum amiodarone levels is sufficiently simple, rapid and reliable to be considered a useful tool in the follow-up of arrhythmic patients chronically treated with amiodarone.