The effect of angiotensin converting enzyme inhibition on myocardial ischaemia was studied in 12 normotensive patients with chronic stable angina and exercise induced ST segment depression. The study was randomised, double blind, placebo controlled, and crossover with treatment periods of two weeks. Enalapril was used to inhibit angiotensin converting enzyme. Assessment was by angina diaries and maximum symptom limited treadmill exercise tests. The results for the whole group showed a significant reduction in systolic blood pressure at rest and at peak exercise. Mean total exercise duration was 466 s (95% confidence interval 406 to 525) when the patients were taking placebo and 509 s (436 to 583) when they were taking enalapril. Four patients prolonged their total exercise time (mean 450 to mean 591 s) by more than 20%. Two patients, however, developed ischaemia earlier on exercise and reduced their total exercise duration (mean 490 to mean 390 s). Although angiotensin converting enzyme inhibition tended to reduce myocardial ischaemia in the group as a whole, some patients improved while others deteriorated. Thus the effects of enalapril are variable and this may have important implications when enalapril is used to treat heart failure in patients with underlying severe ischaemic heart disease.