IN a recent study of the denervated dog heart1, we noted a marked increase in cardiac sensitivity to norepinephrine, a finding anticipated from previous investigations of denervated organs2. Catechol-O-methyl transferase activity was measured in these denervated hearts to determine whether this pharmacological supersensitivity might be accompanied by a decreased ability of the myocardium to O-methylate norepinephrine. The results showed that cardiac denervation produces a decrease in the catechol-O-methyl transferase activity of the myocardium; whether this decrease is related to the phenomenon of supersensitivity, however, is open to considerable doubt.