The cardiovascular effects of an intravenous infusion of adrenaline (10 μg./min.) and isoprenaline (0·75 μg./min.) have been compared in two groups of six patients in the early post-operative period following aortic valvar homograft replacement. These doses were chosen to produce obvious haemodynamic effects without precipitating dysrhythmias. Though both drugs have been shown to improve the circulatory state, adrenaline caused a greater change in stroke volume and in arterial pulse pressure, and for any given rise in heart rate a greater increase in the cardiac index. In addition, adrenaline caused a greater improvement in the relation between oxygen demand and oxygen supply of the body. Isoprenaline did not cause an increase in systolic and pulse pressure and always aggravated hypoxaemia. It is concluded from this study that adrenaline is the more satisfactory drug to use in the early post-operative phase following aortic valvar homograft replacement.