Abstract An assay for plasma norepinephrine and epinephrine levels has been developed by the modification of published procedures. The plasma norepinephrine and epinephrine assay, when compared to currently available methods, provides a substantial decrease in the assay time while providing a 10-fold increase in sensitivity which allows the analysis to be performed on 0.75 ml or less of plasma. Norepinephrine and epinephrine are converted to their O-methylated analogs in the presence of catechol- O-methyl transferase and S-adenosylmethionine-methyl- 3H. Following purification of the labelled normetanephrine and metanephrine by solvent extraction and thinlayer chormatography, the amines are oxidized to vanillin, purified by solvent extraction and counted. The specificity, linearity and precision of the assay are discussed.