The aim of this study was to evaluate the delayed effects of an angiotensin converting enzyme (ACE) inhibitor on blood pressure and on structural and functional alterations in mesenteric small resistance arteries of spontaneously hypertensive rats (SHR). The ACE inhibitor fosinopril (25 mg/kg/day) was administered according to three different schedules: in one group of SHR from 4 to 8 weeks of age (n = 12), in a second group from 8 to 12 weeks of age (n = 15), and in a third group from 4 to 12 weeks of age (n = 12). Eighteen untreated SHR and 18 untreated Wistar-Kyoto rats served as controls. About half the animals in each group were killed at 13 weeks of age, and the remaining were killed at 38 weeks of age. After death, relative left ventricular mass (left ventricular weight/body weight) was calculated. Vascular morphology (media:lumen ratio) and function (responses to norepinephrine and acetylcholine) in mesenteric small resistance arteries were then assessed using a micromyographic technique. Short-term fosinopril, given either before or after the development of hypertension, persistently reduced (but did not normalize) systolic blood pressure, vascular structural alterations, and reactivity to norepinephrine in mesenteric resistance arteries in SHR. These favorable effects were maintained at least for 26 to 30 weeks after treatment withdrawal. The endothelium-dependent vasodilator response to acetylcholine was improved at 13 but not at 38 weeks of age, in treated SHR. Therefore, the vascular response to norepinephrine seems to be dependent mainly on the structure of the vessels, whereas endothelial function is probably more linked to the hemodynamic load.