Inhibition of angiotensin converting enzyme (ACE) in patients suffering from renovascular hypertension results in lowering of the blood-pressure. The development of captopril, an orally active ACE inhibitor and the structure-activity relationship of captopril analogues are described. Some side effects of captopril were attributed to the presence of an SH-group in the molecule. This led to the development of a series of non-mercapto ACE inhibitors derived from captopril. One of them, enalapril, a potent ACE inhibitor with a long duration of antihypertensive action, is now tested in clinical trials.