Abstract A procedure has been developed for the gas chromatographic determination of a structurally new 5HT-uptake inhibitor with antidepressant properties (FG 4963) in plasma and urine. The method involves an extraction from alkaline solution with n-pentane, and a quantitative determination by gas chromatography using an internal standard and a nitrogen-sensitive detector. The binding of FG 4963 to glass during storage and evaporation necessitates the addition of a structurally similar compound to the vials used for collection of the blood and urine, in order to compete for the binding sites of the glass. The method is suitable for pharmacokinetic and clinical studies, the sensitivity being ca. 5 ng/ml in plasma and 1 ng/ml in urine and the precision being 10-15% for concentrations greater than 10 ng/ml.