A sensitive and specific high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) assay for the determination of rivastigmine and its major metabolite NAP 226-90 is presented. A 100 µL plasma aliquot was spiked with a structural analogue of rivastigmine as internal standard (PKF214-976-AE-1) and proteins were precipitated by adding 200 µL of methanol. After centrifugation a volume of 100 µL of the clear supernatant was mixed with 100 µL of methanol/water (30:70, v/v) and volumes of 25 µL were injected onto the HPLC system. Separation was acquired on a 150 × 2.0 mm i.d. Gemini C18 column using a gradient system with 10 mM ammonium hydroxide and methanol. Detection was performed by using a turboionspray interface and positive ion multiple reaction monitoring by tandem mass spectrometry. The assay quantifies rivastigmine from 0.25 to 50 ng/mL and its metabolite NAP 226-90 from 0.50 to 25 ng/mL, using human plasma samples of 100 µL. Validation results demonstrate that rivastigmine and metabolite concentrations can be accurately and precisely quantified in human EDTA plasma. This assay is now used to support clinical pharmacologic studies with rivastigmine. Copyright © 2006 John Wiley & Sons, Ltd.