Abstract The activity of phenylethanolamine N-methyltransferase (PNMT) with noradrenaline and S-adenosylmethionine as substrates was measured in various areas of human brain by high-performance liquid chromatography with electrochemical detection. Commercially available noradrenaline contained about 0.27% adrenaline and was purified for reducing the blank value to increase the sensitivity. Enzymatically formed adrenaline and 3,4-dihydroxybenzylamine (added to the incubation mixture as an internal standard after the reaction) were adsorbed on an aluminum oxide column, eluted with 0.5 m hydrochloric acid and separated by high-performance reversed-phase paired-ion chromatography and measured with electrochemical detection. This assay was very sensitive and PNMT activity was detected in various areas of the human brain including the spinal cord. The enzyme activity was significantly reduced in brain tissues from patients with Parkinson's disease and striatonigral degeneration.