THE following experimental results have been reported by our colleagues on the potentiation of epinephrine action by various substances, clarifying part of its mechanism : (1) The potentiation of epinephrine action on vasoconstriction by certain amino-acids was attributed to a reaction in which the oxidized form of epinephrine gave rise to the oxidative de-amination of amino-acids and was reduced by the generated hydrogen1,2. (2) The epinephrine potentiation in the presence of histidine resulted from its combination with cupric ion in the tissues, which was considered as one of the oxidizing catalysts of epinephrine3.