A high throughput mass spectrometry based method is described for the determination of the 5-HT receptor agonist 311C90, and its desmethyl metabolite, in human plasma. Samples were extracted using the MicroLuteTM system of solid phase extraction in the "96 well' format, automated by means of a robotic sample processor. The extracts were analysed by liquid chromatography tandem mass spectrometry (LC/MS/MS) with thermally assisted electrospray ionization (TurboIonSpray) and selected-reaction monitoring. The LC/MS/MS method offers increased sensitivity, selectivity and speed of analysis over an existing high performance liquid chromotographic method using fluorescence detection.