A method is presented for simultaneously determining diazepam and chlordiazepoxide along with their respective major active serum metabolites N-desmethyldiazepam, and N-desmethylchlordiazepoxide and demoxepam. The drugs are extracted from one ml of buffered serum using chloroform containing 5-(p-methylphenyl)-5-phenylhydantoin as an internal standard. The elution is accomplished using a reversed-phase column with a mobile phase consisting of an acetonitrile/methanol/acetate buffer pH 5.0 (200/225/500) at a flow rate of 2.0 ml/min. Absorbance is monitored at 240 nm using a variable wavelength detector. Each chromatographic separation requires approximately 15 minutes at ambient temperature. Of more than twenty drugs tested for possible interference with this procedure, only methaqualone interferes with the internal standard, and phenytoin with demoxepam.