A method is described for determination of total (free plus conjugated) 3-methoxy-4-hydroxyphenylglycol (MHPG) in plasma using high-performance liquid chromatography with electrochemical detection. After enzymatic hydrolysis of conjugates and deproteinization with zinc sulfate and sodium hydroxide, free MHPG is isolated by extraction prior to isocratic chromatographic separation. The absolute detection limit is about 30 pg which corresponds to a plasma concentration of 2-3 ng/ml when 0.5 ml of plasma is analyzed. Average within-analysis and between-analysis coefficients of variation (6.5 and 8.9%, respectively) are suitable for routine clinical analyses. Severe icterus is without effect, but even mild hemolysis causes detectable losses of MHPG from plasma. The mean +/- S.D. plasma MHPG concentration observed in sixteen healthy individuals (14.5 +/- 3.9 ng/ml) compares favorably with values reported using more complex reference methods.