The daily administration of norepinephrine in oil to dogs, 0.8 to 1.0 mg/kg, over periods of 63 to 82 days results in significant increases in heart weight. Comparable treatment with epinephrine in oil does not produce any significant changes in heart weight. Heart tissue from dogs given norepinephrine in oil for 3 days incorporates significantly greater amounts of 14C from 14C-amino acids into protein than does control tissue. Histologically, myocardial sections from dogs treated for 3 days with norepinephrine in oil show a striking increase in the number of interstitial cells and well defined fatty changes when compared with tissue from control dogs. Myocardial sections from dogs treated with norepinephrine in oil for 63 to 82 days did not reveal either increased interstitial hypercellularity or marked evidence of fatty change.