In the rat adrenalectomy decreases cardiac output, stroke volume, and left ventricular stroke work when these are measured one, three, and seven days after operation. Arterial blood pressure is normal on the first and third days because of a compensatory increase in total peripheral resistance. On the seventh day when the animals were showing obvious adrenocortical insufficiency there was a marked fall in blood pressure. Substitution of 0·9% NaC1 solution for the drinking water improves cardiac function after the first postoperative day. Cardiac output, stroke volume, and stroke work are all greater than in the water maintained, adrenalectomized animals and approach normal levels. The blood pressure on the seventh postoperative day was normal. These animals were apparently healthy, ate well, and were growing.