To determine whether noradrenergic projections to the posterior hypothalamus via baroreflex are altered in the hypertensive state, the extracellular norepinephrine (NE) content of the posterior hypothalamus was measured in both spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) after sinoaortic denervation (SAD) or sham operation. In WKY, blood pressure (BP) and extracellular NE content 24 h after SAD were significantly higher than those of sham-operated rats. Contrarily, SAD did not increase both BP and NE in the posterior hypothalamus of SHR. These results suggest that the noradrenergic pathway via the baroreflex is impaired in SHR. This mechanism may play an important role in the development and maintenance of hypertension in SHR.