Cardiac output of unanesthetized spontaneously hypertensive rats (SHR) and normotensive control rats (NCR) was measured by an aortic pressure pulse contour method. A catheter was introduced into the aortic arch under ether anesthesia and aortic pressure curves were recorded after the recovery from anesthesia. Stroke volume was calculated by reading required pressure and time data on the tracings and substituting them into a special equation. Cardiac output per min per body weight was not significantly different between SHR and NCR. Arterial pressure and total peripheral resistance were higher in SHR than in NCR. On ganglion blockade with hexamethonium bromide in the conscious state, arterial pressure decreased more markedly in SHR than in NCR. Though arterial pressure was still significantly (P less than 0.05) higher in SHR after blockade, cardiac output was larger in SHR more than to account for the difference in arterial pressure. Total peripheral resistance was lower in SHR than in NCR after blockade. It is concluded that, even in the conscious state as in the anesthetized state, the major contribution to the hypertensive state in SHR is an increase in total peripheral resistance due to an elevation of the sympathetic tone.