A sensitive gas chromatographic mass spectrometric assay has been developed to measure plasma 4-hydroxy-3-methoxyphenylethylene glycol and 3,4-dihydroxyphenylethylene glycol. Following addition of deuterated internal standards, the metabolites are propionylated to facilitate extraction with dichloromethane. After evaporation of the solvent, the residue is derivatized with t-butyldimethylsilylimidazole. The reaction product is washed with water, evaporated to dryness, and reconstituted with hexane for gas chromatography mass spectrometry. The single major fragment ion from each of the derivatized metabolites is monitored, yielding a sensitivity of approximately 3-5 pmol ml-1. Improved precision for the 3,4-dihydroxphenylethylene glycol assay may be facilitated by synthesis of a more stable internal standard.