Trans-oesophageal ultrasonic monitoring of aortic blood flow was found to offer a system allowing beat to beat observation of the total peripheral resistance, without the need of a very sophisticated calibration technique. Experimental evidence from dogs indicates a very good correlation between the total peripheral resistance and the proportion of the stroke volume flowing down the aorta during diastole. There was also a very good negative correlation between the maximum systolic velocity and the total peripheral resistance. This combination would suggest that the elastic properties of the proximal aorta play an important role in reducing the instantaneous work load on the myocardium.