Arterial blood pressure and heart rate were measured in 43 patients with acute myocardial infarction and a systolic blood pressure greater than or equal to 120 mmHg during sublingual administration of 5 mg of isosorbide dinitrate. In 25 of them right heart haemodynamics were also measured. Severe (greater than or equal to 25%) hypotension developed in 12 patients (Group 1, systolic blood pressure 158 +/- 28 to 78 +/- 17 mmHg, mean +/- SD) but not in the remaining 31 (Group 2) and was accompanied by a fall in heart rate (82 +/- 20 to 70 +/- 22 beats min-1, P less than 0.05), in cardiac output (4.3 +/- 0.3 to 3.2 +/- 0.41 min-1, P less than 0.02, n = 5) and in systemic vascular resistances (2326 +/- 463 to 1532 +/- 442 dynes sec-1 cm.5, P less than 0.02) not present in Group 2. The reduction in right (Group 1, 8 +/- 3 to 3 +/- 1, vs. Group 2, 10 +/- 3 to 6 +/- 3 mmHg, P less than 0.005) and in left ventricular filling pressures (Group 1, 15 +/- 4 to 8 +/- 2, vs. Group 2, 18 +/- 6 to 13 +/- 5 mmHg, P less than 0.001) was more remarkable in Group 1. In this group there was also a high incidence of anterior infarction (9/12, 75%).(ABSTRACT TRUNCATED AT 250 WORDS)