Cocaethylene (benzoylecgonine ethyl ester or ethyl cocaine) is a transesterification product of cocaine and ethanol that has been observed in the urine of individuals using these drugs in combination. There is evidence that cocaethylene is pharmacologically active, and its formation in vivo may contribute to the toxicity of cocaine. A new method is presented here which enables the quantification of cocaethylene and cocaine, as well as the cocaine metabolites benzoylecgonine and norcocaine in liver tissue. This method utilizes high-performance liquid chromatography with uv detection (235 nm), and the propyl ester of cocaine is used as an internal standard. Liver homogenates are first buffered with 0.1 N dibasic potassium phosphate (pH 9.1) and then extracted with methylene chloride:isopropanol (9:1). Extraction efficiencies were approximately 75-85% for the compounds of interest. The coefficient of variation for replicate determinations (N = 10) of cocaethylene concentration was 5.75%, with comparable values obtained for cocaine, norcocaine, and benzoylecgonine. The detection limit for cocaethylene, based on a peak height threefold greater than background noise, was approximately 1.7 ng of injected compound. Using this method, it was demonstrated that cocaethylene is present in mouse liver following cocaine and ethanol administration, with an apparent rapid rate of formation and elimination.