We measured regional cerebral blood flow (rCBF) in young and old Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats by a minimally invasive microsphere technique. Blood flows to the cerebrum, diencephalon, mesencephalon, cerebellum and pons-medulla during normocapnia were determined. To test the ability of the vessels to dilate, rCBF was also measured during hypercapnia. Reactivity to CO2 was calculated as delta rCBF/delta paCO2. In the old SHR, blood flow to the pons-medulla (88 +/- 8 ml/min/100 g, mean +/- SEM) was markedly lower than that in the young SHR (107 +/- 4 ml/min/100 g, p < 0.05), whereas the difference of those values in the old and young WKY rats was slight (0.05 < p < 0.1). There were no differences in the values of blood flow to the cerebrum, diencephalon, mesencephalon or cerebellum between the young and old rats in both species. Cerebrovascular CO2 reactivity was markedly impaired in the old SHR (p < 0.05) compared to that in the young SHR, but the difference in reactivity in the WKY rats was not significant. The results indicate that blood flow to the pons-medulla is reduced in association with age, particularly in the hypertensive animals, and that the ability of the cerebral vessels to dilate is impaired homogeneously by the combination of chronic hypertension and aging.