NOREPINEPHRINE is present in various organs of the body and in the blood plasma1. The adrenal medulla is not necessary for the maintenance of tissue stores of this catecholamine1, and it is likely that synthesis of norepinephrine occurs in the sympathetic nervous tissue throughout the body. Norepinephrine in the plasma originates from discharge of the neurohumour from the sympathetic nerve endings. The circulating catecholamine is excreted, metabolically inactivated (predominantly by O-methylation), or rebound in the tissues2,3. A portion of the norepinephrine present in each organ is derived from the circulating pool of catecholamine. The ability of the heart to take up, concentrate, and store intravenously administered norepinephrine has been amply demonstrated4–6. It has recently been shown that the isolated perfused heart is capable of synthesis of norepinephrine from tyrosine, at a sufficiently rapid rate to account for a considerable portion of the norepinephrine in this organ7. Thus norepinephrine stores in the heart could be maintained in part by extraction of the catecholamine from the circulation and in part by synthesis. We have used an isotope dilution technique to demonstrate that in the normal rat about 60 per cent of the norepinephrine present in the heart is synthesized in this organ, while the remainder is derived from the circulating pool of norepinephrine