Arterial pressure, cardiac output, and total peripheral resistance were compared between spontaneously hypertensive rats (SHR) and normotensive control rats (NCR) at 4, 7, and 12 months of age. The rats were anesthetized with pentobarbital sodium, thoracotomized and kept under positive pressure respiration. Cardiac output was measured with an electromagnetic flowmeter probe placed at the ascending aorta. On all the age groups, arterial pressure and total peripheral resistance were significantly higher in SHR than in NCR, while cardiac output per body weight was not different. However, the significant differences in pressure and resistance at each age disappeared after ganglion blockade with hexamethonium bromide. Total peripheral resistance was not higher in SHR than in NCR even after cardiac output, which had once been diminished by blockade, was restored to the pre-blockade level by dextran infusion. Phenoxybenzamine, an alpha adrenergic receptor blocker, also equalized arterial pressure and total peripheral resistance between SHR and NCR, both aged about 10 months, without decrease in cardiac output. It is concluded that, in both young and old SHR's, the hypertensive state is maintained by an increase in total peripheral resistance ascribable to sympathetic activity.