Studies on the effect of antihypertensive agents on resistance arteries in hypertensive patients have in the past yielded inconclusive results regarding the ability of these drugs to induce a regression toward normal of either the structure or the function of these critically important vessels. We have recently compared the effects of the angiotensin-I-converting enzyme inhibitor cilazapril and of the beta blocker atenolol on the structure and the function of subcutaneous resistance arteries of essential hypertensive patients. The patients were randomly assigned to receive either cilazapril or atenolol for a period of 2 years. The blood pressure was normalized for the duration of the trial by both drugs. The media-to-lumen ratio of resistance arteries, which was significantly increased in all hypertensive patients before starting treatment, was normalized by the 2-year treatment with cilazapril, whereas treatment with atenolol did not result in any change in this vascular parameter. Treatment with cilazapril also returned to normal the contractile responses to several vasoconstrictors, particularly endothelin 1. Endothelium-dependent relaxation responses of blood vessels to acetylcholine were abnormal in hypertensive patients and improved in the cilazapril-treated patients, but remained unchanged in the atenolol-treated ones. We conclude that treatment with the angiotensin-I-converting enzyme inhibitor cilazapril corrects in part the vascular remodeling and the functional abnormalities of resistance arteries of hypertensive patients, whereas treatment with the beta blocker atenolol does not. These results may indicate that treatment with cilazapril and perhaps with other angiotensin-I-converting enzyme inhibitors as well may improve the clinical outcome in hypertension by inducing a regression of abnormal resistance vessel structure and function.