The hypothalamus contains two distinct zones which affect the activity of myocardial and vascular muscle. The median ventricular nucleus stimulates the liberation of peripheral catecholamines and, by so stimulating the adrenoreceptors, opens up the slow calcium canals. The resulting stimulation of the myocardial and vascular muscle cells causes tachycardia, and an increase in myocardial contraction and in peripheral resistance. The paraventricular nucleus antagonises these effects by inhibiting peripheral catecholamine secretion so blocking or slowing the passage of Ca++ through the slow calcium canals. The study of effective, active concentration of D 600, a powerful calcium antagonist, shows that the paraventricular nucleus is not a direct calcium antagonist like D 600 whose effects are not potentiated, and confirms that it acts indirectly by inhibiting peripheral noradrenaline secretion.