The use of liquid chromatography-tandem mass spectrometry (LC-MS-MS) for the quantitative analysis of an 'atypical' antipsychotic agent in human plasma is described. The method uses atmospheric pressure chemical ionisation, and offers increased sensitivity, selectivity and speed of analysis over an existing high-performance liquid chromatography method using fluorescence detection. This method enabled same day turn around of results for in excess of 100 samples, including sample preparation, data acquisition and processing. LC-MS-MS was also used to detect and characterise known and unknown in vivo metabolites of the drug in human urine and plasma, using electrospray ionisation.