This is a reversed-phase liquid-chromatographic method, with electrochemical detection, for simultaneously measuring, in plasma, the concentrations of the catecholamine precursor dihydroxyphenylalanine (DOPA); the endogenous catecholamines norepinephrine, epinephrine, and dopamine; and the deaminated catecholamine metabolites dihydroxyphenylacetic acid (DOPAC) and dihydroxyphenylglycol (DHPG). We used this method to assess effects of monoamine oxidase (EC 1.4.3.4) inhibition in humans. Plasma DHPG concentrations as determined by the present method (mean 826, SEM 61 ng/L) were similar to those found by other methods. Inhibition of monoamine oxidase (by administering deprenyl or tranylcypromine) decreased plasma DHPG by greater than 65%, plasma DOPAC by greater than 50%, and plasma DOPA by about 20%, without consistently affecting norepinephrine or epinephrine. Simultaneous measurement of DOPA, catecholamines, and DHPG may be useful for examining the synthesis, release, and intraneuronal metabolism of norepinephrine. The assay method is rapid, reliable, and simple, and it provides a more comprehensive assessment of noradrenergic nervous function than does measurement only of catecholamines.