A simple high-performance liquid chromatographic procedure for quantifying lidocaine and nine of its metabolites in human plasma and urine is described. This method involves aqueous acetylation of the metabolites with acetic anhydride under mild basic conditions (pH approximately 8.5). Lidocaine and the derivatized metabolites are extracted under basic conditions with ethyl acetate. Calibration curves constructed for these compounds are linear in the concentration range studied (0.2-500 micrograms/ml). The relevance of this method is shown by measuring the plasma and urine profiles of two patients with suspected myocardial infarction.