Cocaine ethyl ester (“cocaethylene”) is a cocaine metabolite formed by hepatic carboxylesterases in the presence of ethanol. In the human liver, this pharmacologically active cocaine analog may reach concentrations similar to those of cocaine. To further investigate the intrinsic hepatotoxic potential of cocaethylene and to compare its effects with the known hepatocyte-damaging effects of cocaine, primary short-term cultures of hepatocytes derived from phenobarbital-pretreated rats were exposed to cocaine or cocaethylene (10 to 1000 μmol/L). Both drugs caused time- and concentration-dependent release of lactate dehydrogenase into the extracellular medium in a congruent pattern. Spontaneous hydrolysis of cocaethylene was negligible at pH 7.4, and the rate of 2-diethylaminoethyl-2,2-diphenylvalerate (SKF-525A) and phenylmethylsulfonylfluoride-sensitive enzymatic degradation in hepatocyte cultures was similar to that of cocaine (half-time, ≈ 3 hr). Furthermore, both cocaine and cocaethylene induced massive NADPH-dependent reactive oxygen species production in hepatic microsomal suspensions and in homogenates of cultured hepatocytes, as directly demonstrated by superoxide dismutase– and catalase-sensitive luminol chemiluminescence. The integrated photon emission and the maximal slopes of the luminol chemiluminescence displayed similar concentration-response curves for cocaine and cocaethylene. These data suggest that reactive oxygen species, generated during the interaction of cocaine and cocaethylene with cytochrome P-450, may be the common mediators of hepatotoxicity induced by these compounds. Moreover, they indicate that the potential for cocaethylene to induce acute lethal cell injury in hepatocytes through oxidative mechanisms is similar to that of its parent compound, cocaine. (HEPATOLOGY 1993;18:1154-1161).