A gas cromatographic method for the determination of the benzothiazepine diltiazem together with its major metabolite desacetydilitiazem, is described. Silylation of the desacetyl derivative separates the metabolite from the parent drug on a 1% OV-17 column and cyclopam is used as an internal reference standard. The compounds are analysed by means of a nitrogen detector which allows the determination of 10 ng/ml of both compounds in plasma. The method has been used to determine both diltiazem and its desacetyl derivative in plasma obtained from healthy volunteers after oral doses of 60-210 mg of diltiazem.