A rapid and reliable high-performance liquid chromatographic (HPLC) assay has been developed for the measurement of the antiarrhythmic drug, amiodarone (A), and its metabolite, N-desethyl-amiodarone (NDA), in serum. The procedure involves addition of absolute ethanol to serum, previously adjusted to pH 6, followed by centrifugation. The supernatant is analyzed by reverse-phase HPLC with ultraviolet-visible detection. There is no intereference from endogenous substances or other commonly used antiarrhythmic agents. The mean overall accuracy of the HPLC assay is 96.9 +/- 9.1 (SD)% (n = 37) for A over the concentration range 0.10-10.00 micrograms/mL, and 99.8 +/- 4.8 (SD)% (n = 32) for NDA over the concentration range 0.10-7.50 micrograms/mL. The within-day coefficient of variation is less than 7% for A and NDA. The lower limit of quantitative sensitivity is 0.10 micrograms/mL for A and NDA, and the lower limit of qualitative detection is 1.0 ng for both compounds.