A new assay method for the determination of free and total 3-methoxy-4-hydroxyphenylethylene glycol (MHPG) in human plasma is described. MHPG was purified with a Bond Elut PH column followed by ethyl acetate extraction. High-performance liquid chromatography (HPLC) with electrochemical detection was used for separation and detection of MHPG. Total MHPG was measured after enzymatic hydrolysis with sulfatase type H-5. 3-Hydroxy-4-methoxyphenylethylene glycol (iso-MHPG) was used as an internal standard to correct the recovery of extraction. One assay could be completed within 20 min with a short reverse-phase column. This technique is sensitive, reliable and less time-consuming than other HPLC methods. With this method, the plasma values of MHPG in healthy controls were in good agreement with those using gas chromatography-mass spectrometry.