The comparative haemodynamic effects of transdermal and intravenous nitroglycerin were evaluated in 16 patients with haemodynamic and radiographic left heart failure following a recent myocardial infarction. After the control period patients were randomized to transdermal (10 mg(24 h)-1) or intravenous (mean dose: 40 +/- 9 micrograms min-1) nitroglycerin. Haemodynamic parameters were recorded after 0.5,1,2,6,12,18 and 24 h during administration of the drug and 2 h after drug discontinuation. After the washout period the alternate system of nitroglycerin administration was adopted, according to a cross-over design. No differences were found in baseline measurements. Transdermal nitroglycerin reduced the pulmonary artery wedge pressure after 0.5 h (from 21 +/- 5 to 16 +/- 5 mmHg; P less than 0.05). The peak effect occurred at 2 h (12 +/- 5 mmHg). The improvement was sustained over 24 h. Transdermal nitroglycerin also significantly reduced mean pulmonary arterial and right atrial pressures (from 28 +/- 3 to 20 +/- 5 mmHg and from 6 +/- 3 to 2 +/- 2 mmHg at peak effect, respectively). Cardiac index increased from 2.5 +/- 0.6 to 2.8 +/- 0.8 l min-1 m-2 (P less than 0.05). There was no change in heart rate. Similar haemodynamic changes were observed after the intravenous infusion of nitroglycerin. Thus transdermal nitroglycerin is a safe and effective treatment of acute myocardial infarction with signs of left ventricular failure when an intravenous nitroglycerin infusion cannot be properly implemented.