Fourteen patients with acute myocardial infarction were given 0.3 mg sublingual nitroglycerin within the first 12 hours of their acute myocardial infarction. Five minutes after sublingual nitroglycerin mean arterial pressure fell 9 mmHg (1.2 kPa) and remained significantly reduced for 30 minutes. Pulmonary capillary wedge pressure fell from a mean control value of 17 to 12 mmHg (2.3 to 1.6 kPa) and also remained reduced for 30 minutes. Heart rate was significantly raised and stroke work index reduced at five minutes. Patients with a stroke work index of greater than 55 g m per m-2 b.s.a. responed to nitroglycerin with a fall in both pulmonary capillary wedge pressue and strokework index while in those with a stroke work index of less than 55 g m per m-2 b.s.a. stroke work index did not fall concomitantly with the fall in pulmonary capillary wedge pressure. In one patient, nitroglycerin led to a precipitious fall in arterial pressure andrecurrence of chest pain.