By means of an electromagnetic flowmeter, cardiac output was measured simultaneously with arterial pressure in spontaneously hypertensive rats (SHR, Okamoto strain) anesthetized with pentobarbital sodium under positive pressure respiration. The rats were aged 12-23 weeks and in the early stage of hypertension. Cardiac output, arterial pressure, and total peripheral resistance were 12.6±2.7ml/min/100Gm, 116±24.6mmHg, and 9.46±2.74mmHg/(ml/min/100Gm) respectively, each being the mean of 9 rats with SD. When compared with normotensive control rats (NCR), cardiac output was not different, while arterial pressure (P<0.005) and total peripheral resistance (P<0.025) were significantly higher in SHR. After ganglion blockade with hexamethonium bromide, there was no difference in the above 3 parameters between SHR and NCR. Heart rate was also significantly higher in SHR before ganglion blockade and not so after blockade.