A gas chromatographic assay procedure was developed to quantitate the reduction product of disulfiram, diethyldithiocarbamate (DDC), in blood and plasma. The procedure involved the in situ methylation of DDC prior to the extraction and chromatography of the methyl ester. The minimal sensitivity achieved was 0.2 .mu.g/ml from 1 ml of blood or plasma. The coefficient to variation about any concentration was 10.5%. Calibration curves having a reproducible nonlinear form were prepared up to 9 .mu.g/ml. The assay procedure was used to evaluate the stability of disulfiram and DDC in blood. Disulfiram was rapidly and quantitatively reduced to DDC within 4 min. The DDC thus formed decomposed in human and dog blood with half-lives of 70 and 100 min, respectively. Disulfiram, which has an ethanol-sensitizing effect, is used as part of an aversion therapy for treating alcoholics.