A sensitive, precise and accurate method for simultaneous quantitation of lidocaine and its deethylated metabolites by gas chromatography-mass fragmentography has been developed. Propyl derivatives of the deethylated metabolites are formed directly in either plasma or urine by treatment with propionaldehyde and sodium cyanoborohydride. The propyl derivatives and unchanged lidocaine are extracted, separated by gas chromatography and quantitated by mass fragmentography using mepivacaine as the internal standard. Quantitation of these compounds to levels as low as 50 ng/ml body fluid has been achieved with coefficients of variation less than 10%.