It is confirmed by several studies that in normal subjects a substance recognized by antibodies anti digoxin exists. Such a substance can be found at increased concentration in pregnant women, neonates, in liver or kidney diseases. A limited increase in concentration has been also registered in patients with essential hypertension and in normotensive patients with a family history of hypertension. Serum or urines rich in such a substance show an increased capacity of inhibiting in vitro the sodium-potassium pump and therefore in reducing also in vivo the capacity of reabsorption of sodium and with it, of water. The investigators interest for this substance has two main reasons: 1) the interference that such a substance has in dosages of digitalis in therapeutic monitorizing; 2) the possibility that such a substance has an important physiological role in hydroelectrolytic metabolism.