Plasma concentrations of 5-(2-chlorophenyl)-7-ethyl-1,3-dihydro-1-methyl-2H-thieno[2,3-e][1,4]diazepin-2 -one (clotiazepam, CTZ, Trecalmo), a thienodiazepine anti-anxiety agent, and two of its metabolites (hydroxy- and desmethyl-clotiazepam) are reliably quantified by electron capture GLC. After addition of diazepam as internal standard, 1-ml plasma samples are extracted with benzene-isoamyl alcohol (98.5:1.5). The organic extract is evaporated to dryness, reconstituted, and directly chromatographed using a 3% SP-2250 liquid phase at 260 degrees C. Sensitivity limits are 1-3 ng of each of the 3 compounds per ml of plasma. The method is applicable to clinical pharmacokinetic studies in humans. After p.o. ingestion of a single 5-mg dose of clotiazepam by three healthy volunteers the drug was rapidly absorbed. Values of elimination half-life were 6.5, 7.0, and 17.8 h, respectively. The two metabolites appeared and disappeared approximately in parallel with the parent compound. Clotiazepam was more than 99% bound to plasma protein.