We previously reported that the response of the arterioles in spontaneously hypertensive rats (SHR) to histamine is blunted compared with that in normotensive control rats (Wistar-Kyoto rats [WKY]). The present study was designed to analyze the extent to which this blunted arteriolar response may be attributed in SHR to the concurrent elevation of circulating glucocorticoids through the use of adrenalectomy with and without dexamethasone supplementation. Mesenteric arterioles were observed by intravital microscopy under general anesthesia, and their lumen diameters were measured after histamine superfusion. The concentration-response curve with histamine was compared with that of an endothelium-independent vasodilator, sodium nitroprusside. At the end of each experiment, papaverine was applied topically to determine the maximal diameter for each vessel, from which a measure of arteriolar tone could be computed. The arteriolar tone in sham-operated SHR is set at a higher steady-state level than in sham-operated WKY. The concentration required for a 50% dilator response (EC₅₀) of histamine in adrenalectomized SHR was restored to the level of WKY. Adrenalectomy did not significantly affect the EC₅₀ of histamine in WKY. When adrenalectomized SHR received a supplement of dexamethasone, the arteriolar response was found to show the same refractory pattern to histamine as sham-operated SHR. In contrast, the EC₅₀ of sodium nitroprusside in sham-operated and adrenalectomized SHR was similar to that in sham-operated WKY. Our results indicate that the impaired dilator response to histamine in SHR is related to an enhanced adrenal glucocorticoid secretion.