With regard to the increasing importance of determining plasma levels of antidepressant drugs a procedure has been worked out which makes it possible to analyze these substances in a simple and sensitive way. 5 ng )ca. 20 pmol) amitriptyline, nortriptyline, imipramine, desmethylimipramine, chlorimipramine, desmethylchlorimipramine, and protriptyline can be detected with a high-pressure liquid chromatograph and a UV-monitor without prior derivatisation. The extraction is made from 1 ml serum. Because of their retention time the substances can be identified. This fact allows simultaneous analyzation of all tricyclic antidepressants.