Triiodothyronine (T3) potentiated epinephrine-induced contractions of isolated rabbit aortic strip only under the following conditions: (1) copper acetate (2 x 10-7) addition to the bath or (2) when trace metal contaminants were occasionally present in the bath, as suggested by short duration control responses to epinephrine. EDTA (ethylenediaminetetraacetate), a potent chelating agent, similarly potentiated epinephrine contractions. It was further demonstrated that T3 delays the metal-catalyzed oxidation of epinephrine in separate incubate experiments. This effect appears to be the major factor in the potentiation of epinephrine contractions by T3. It is further concluded that T3 probably acts by virtue of its metal chelating effect.