The effects of isosorbide dinitrate on acute myocardial ischaemia were studied in 19 patients during coronary angioplasty. The duration of balloon inflation to the onset of ST segment depression was increased by the administration of intracoronary isosorbide dinitrate from (mean +/- SEM) 13 +/- 2 to 20 +/- 2 s (48%) (P less than 0.01) and the duration to 1 mm ST segment depression increased form 23 +/- 2 to 29 +/- 3 s (28%) (P less than 0.01). Systolic blood pressure fell from 118 +/- 3 to 111 +/- 4 mmHg but there was no change in intracoronary distal occlusion pressure. Thus during coronary occlusion isosorbide dinitrate prolongs the time to the onset of myocardial ischaemia.