Experiments were performed in anesthetized 18-19-week-old spontaneously hypertensive rats (SHR) to evaluate the effects of delayed antihypertensive treatment on cerebrovascular function. Animals were treated for 25 +/- 1 days with an oral antihypertensive regimen consisting of hydralazine, reserpine, and chlorothiazide, resulting in normotension within 2 weeks. Cerebral arterioles were examined via a constantly suffused open cranial window and video microscopy. Resting cerebral blood flow was measured using radioactive microspheres and the reference organ method. While untreated SHR exhibited reductions in arteriolar diameter compared with normotensive Wistar Kyoto rats (WKY), treatment restored arteriolar dimensions to normal. Increments in microvascular wall area, associated with medial hypertrophy in untreated SHR, were completely reversed in treated SHR to a magnitude not different from control. Resting cerebral blood flow was, however, decreased in treated SHR compared with both untreated SHR and WKY; this was due to an increase in total cerebrovascular resistance compared with WKY. Additionally, microvascular pressure in the largest arterioles in treated SHR was reduced compared with both WKY and untreated SHR. There was a significant increase in the relative pressure drop accounted for by the arterial vessels upstream from the cerebral microcirculation in treated SHR. These results suggest that 1) cerebral microvascular abnormalities induced by chronic hypertension are reversed by delayed antihypertensive therapy, and 2) there is a persistent elevation in cerebrovascular resistance upstream from the microcirculation representing large vessel adaptations that may not be readily reversible with treatment.