The effect of the angiotensin-converting enzyme (ACE) inhibitor quinapril on myocardial ischaemia was tested in a randomized double-blind cross-over study of 16 men (mean age 62 [44-75] years) with angiographically demonstrated coronary heart disease, exercise-induced ST-segment depression and stable angina. Exercise ECGs were recorded before and 4 hours after a single dose of 10 mg quinapril and after a 14-day course of 10 mg quinapril twice daily. The single dose decreased the average ST-segment depression from 1.12 mV (placebo) to 0.74 mV (P less than 0.05); after 14 days on quinapril the ST-segment depression decreased from 0.91 mV (placebo) to 0.72 mV (P less than 0.05). While heart rate remained unchanged the average resting arterial blood pressure fell from 136/80 to 120/74 mmHg (P less than 0.05) after a single dose and from 141/83 to 127/78 mmHg (P less than 0.05) after 14 days on the drug. These data indicate that ACE inhibitors should be considered as a means of treating coronary heart disease and angina.