Results from studies using pump-perfused rat hindquarters are consistent with increased wall-to-lumen ratios in resistance vessels of spontaneously hypertensive rats (SHR). However, in vivo measurements of cremaster arterioles have not shown increased wall-to-lumen ratios in SHR. To investigate this discrepancy, we studied three groups of male SHR and Wistar-Kyoto rats at 12 weeks of age. In the first two groups, the cremaster muscle was prepared to allow microscopic observation while the hindquarters were pump-perfused with increasing concentrations of norepinephrine in oxygenated Tyrode's solution. Both groups of SHR showed an increase in vasodilated resistance and elevated maximal vasoconstrictor response. In the first group, arterioles showed dose-dependent constriction that was greater in smaller arterioles but did not differ between hypertensive and normotensive rats. Vasodilated diameters of second-order arterioles were significantly smaller in the hypertensive rats. In the second group, servo-null pressures in the first-order arteriole showed that the microvessels contributed proportionally to the elevation in resistance in both SHR and normotensive rats. In the third group, first- and second-order arterioles were measured in vivo and histologically. Arteriolar diameters did not differ between SHR and normotensive rats with either method. In fixed sections the cross-sectional area of the media-intima was greater in the SHR. Therefore, data from the pump-perfused rat hindquarters accurately reflect vasoconstrictor responses of the arterioles, and in deference to in vivo measurements on arteriolar walls that include the adventitia, the increased response in the SHR can be explained by hypertrophy of the arteriolar medial-intimal area.