Seventeen male untreated mild essential hypertensive patients aged 41±2 years agreed to participate in a double-blind randomized trial to test the effects of antihypertensive treatment on the structure and function of subcutaneous resistance arteries. Patients were treated with either 50 to 100 mg/d atenolol or 2.5 to 5 mg/d cilazapril. Blood pressure before treatment was 148±6/99±1 and 147±2/99±1 mm Hg, respectively. At 1 year of treatment blood pressure was 131±4/85±2 and 132±2/87±1 mm Hg, respectively. Resistance arteries (200 to 400 μxm lumen diameter) dissected from subcutaneous gluteal biopsies obtained before treatment and at 1 year showed that the media-lumen ratio of arteries from patients treated with cilazapril was reduced to 6.31±0.21% from 7.54±0.31% before treatment (P<.05), still slightly but significantly larger (P<.05) than the media-lumen ratio of resistance arteries of normotensive control subjects (5.15±0.30%). In contrast, in arteries from patients treated with atenolol there was no significant change with treatment (7.97±0.60% before and 8.07±0.45% after 1 year of treatment). Active wall tension responses to endothelin-1 were blunted in hypertensive patients and normalized in the cilazapril-treated patients. Depressed active media stress responses to norepinephrine, arginine vasopressin, and endothelin-1 were accordingly normalized in the patients receiving cilazapril as the media width became thinner but were unchanged in those taking atenolol. These results suggest that treatment for 1 year with the converting enzyme inhibitor cilazapril corrects in part the structural and functional abnormalities present in subcutaneous resistance arteries of patients with mild essential hypertension.