The authors describe a method for the plasma determination of mianserin by gas--liquid chromatography (GLC), using cyproheptadine as an internal standard. After extraction from 2 ml of plasma by a hexane:isoamyl alcohol (99:1, vol/vol) mixture and its purification by reextraction, the mianserin is determined by GLC, using a glass column packed with 3% OV1 on Gas-Chrom Q, and a nitrogen-sensitive detector. Since the between-day coefficient of variation (n = 15) is less than 8%, this technique appears to be sufficiently reproducible for routine determination in therapeutic concentrations (detection limit with accuracy = 2.5 microgram/L). As an example, the authors describe the time variations of the plasma concentration of mianserin after a first-dose ingestion of 30 mg. Preliminary studies of kinetic parameters are determined in 11 adult patients following a single oral dose (30 mg of mianserin).