The immediate haemodynamic effects of prenalterol and nitroglycerine were examined in 15 patients, with severe chronic heart failure. Prenalterol was given intravenously in increasing doses of 2, 4, and 8 mg. Cardiac index increased significantly by 16%, 24%, and 32%, respectively. Heart rate increased by 16%, 19%, and 24%. Stroke volume index, systemic artery pressure, pulmonary artery pressure, and right atrial pressure did not change. Prenalterol reduced systemic vascular resistance by 15%, 17%, and 24%, respectively. Forearm blood flow and forearm vascular resistance was unchanged. Cardiac index and heart rate were not changed by 0.5 mg nitroglycerine, administered sublingually. Systolic and diastolic blood pressure were on average reduced by 14% and 12%, respectively. Systolic and diastolic pulmonary artery pressure and right atrial pressure were similarly reduced by 17%, 31%, and 39%, respectively. Nitroglycerine lowered calculated systemic vascular resistance by 11%, whereas forearm blood flow and forearm vascular resistance was unchanged. The conclusion is that prenalterol acutely increased cardiac index and improved haemodynamics in 14 out of 15 patients, mainly due to an increased heart rate. Nitroglycerine did not change cardiac index in the same group of patients.