This is an interim report of haemodynamic studies with oral muzolimine in male patients under 70 yr with haemodynamically confirmed left ventricular failure complicating acute myocardial infarction. All were in sinus rhythm and without hypotension (SBP 100 mmHg) and were studied by conventional intravascular haemodynamic methods within 18 hr of onset of symptoms of acute myocardial infarction. A preliminary dose-finding study in four patients indicated that both 60 and 120 mg oral muzolimine induced substantial diuresis but only the latter dose was associated with an appreciable reduction in left heart filling pressure within 2 hr. The aims of the main study are to determine the haemodynamic effects of oral muzolimine 120 mg given once daily for 10 days in these high risk patients. Thereafter patients are randomised to continued oral muzolimine or placebo for a period of three months. The effects of such treatment on their haemodynamic profile at rest and during dynamic exercise are being measured by comparison with the 10 day measurements. In the first six patients entered into the main study, 120 mg oral muzolimine resulted in a progressive reduction in the elevated left heart filling pressure without tachycardia or any other major haemodynamic change over 8 hr on both Day 1 and Day 2. These potentially advantageous haemodynamic effects await confirmation from further results in this on-going study.