The haemodynamic effects of a new beta 1-receptor agonist, 1-(4 hydroxyphenoxy) 3-isopropylamino-2-propanol, were studied in 25 patients after acute myocardial infarction using non-invasive methods. The drug caused an increase in systolic blood pressure and pulse pressure, without change in diastolic blood pressure, and a slight increase in heart rate and reduction in the pre-ejection period. These changes were greater in patients without a history of left heart failure. It is suggested that this cardioselective drug possesses positive inotropic activity but only slight positive chronotropic activity. The substance has been further investigated as a possible antidote to unwanted cardiac side effects of the cardioselective beta-blocker, metoprolol. The changes in the cardiovascular dynamics caused by metoprolol in patients with acute myocardial infarction were promptly reversed by this new beta 1-agonist. With its positive inotropic properties and its efficacy in reversing the effects of a cardioselective beta-blocker, the drug is a potentially useful pharmacological agent to support an acutely depressed myocardium in patients on beta-blocking agents.