Incubation of [14C]lidocaine with rat liver microsomes in the presence of an NADPH-generating system resulted in covalent bindings of a 14C-labelled material to microsomal protein. The covalent binding of radioactivity needed NADPH and atmospheric oxygen, and was diminished by purging of carbon monoxide and the addition of SKF-525A. Hence the covalent binding of a 14C-labelled material resulting from a reactive metabolite of lidocaine formed by cytochrome P450-dependent monooxygenation. The covalent binding measured at various concentrations of lidocaine (2.5-30 microM) followed Michaelis-Menten kinetics, and the Km value (4.52 microM) of the activation reaction was close to the Km value (1.78 microM) of lidocaine 3-hydroxylation. The metabolism-dependent covalent binding of lidocaine to microsomal protein as well as lidocaine 3-hydroxylase activity was much lower in the Dark Agouti strain rat, which is known as a poor-metabolizer animal model of debrisoquine 4-hydroxylation, than in the Wistar rat for the corresponding sexes. The covalent binding in male rats was greater than that in females of both strains, but the extent of the sex difference in the binding was smaller than that of the lidocaine N-deethylase activity in Wistar rats. Propranolol and quinidine, specific inhibitors of debrisoquine 4-hydroxylase, markedly inhibited lidocaine 3-hydroxylase activity of Wistar male rats, but not N-deethylase activity. These compounds also inhibited the metabolism-dependent covalent binding of lidocaine to microsomal protein. These strain difference and inhibition studies showed that the reaction converting lidocaine to a reactive metabolite capable of binding covalently to microsomal protein was related to lidocaine 3-hydroxylation, and may be catalysed by cytochrome P450 isozyme(s) belonging to the CYP2D subfamily. The covalent binding of radioactivity to rat liver microsomal protein was diminished by nucleophiles, reduced glutathione and cysteine, indicating that the reactive metabolic intermediate of lidocaine is an electrophilic metabolite such as an arene oxide.