A new modification of the trihydroxyindole method for the fluorometric estimation of mixtures of epinephrine and norepinephrine is described. It offers improved sensitivity and discrimination and has a low and stable reagent blank. Its distinctive features are: (1) β-mercaptoethanol is used as reducing agent, (2) the fluorescent solutions are reacidified to approximately pH 5 after alkaline rearrangement to the trihydroxyindole configuration, (3) cupric ions are used as catalysts for the oxidation at pH 3. © 1968.