A gas chromatographic method for the determination of oxpentifylline and a metabolite, 1-(5'-hydroxyhexyl)-3,7-dimethylxanthine is described. Oxpentifylline, metabolite and internal standard are extracted from basified plasma into dichloromethane, then the metabolite and internal standard are converted to their O-trifluoroacetates. Analysis by gas--liquid chromatography using a nitrogen-selective detector allows quantification of oxypentifylline and 1-(5'-hydroxyhexyl)-3,7-dimethylxanthine down to levels of 3 ng/ml and 3--10 ng/ml, respectively. The assay had been applied to plasma samples from volunteers after both intravenous and oral administration of oxpentifylline. The need to separate plasma from erythrocytes immediately after venipuncture sampling to prevent further metabolism of oxpentifylline is emphasized.