The characterization of alpha-adrenoceptor-mediated contractile responses and the effects of the calcium channel blocker nifedipine on these responses were investigated in mesenteric arterial strips from 13-week-old male spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Contractile responses to the alpha-adrenoceptor agonists phenylephrine and clonidine were mediated through the activation of alpha 1-adrenoceptors. The dose producing a half-maximum response (ED50) for the agonists was higher in SHR than in WKY. Affinities of alpha 1-adrenoceptors were similar between the two strains. When arterial strips from both strains were treated with the same concentration of phenoxybenzamine, the maximum response to each agonist was weaker in SHR. The alpha 1-adrenoceptor occupancy-response relationship for phenylephrine was hyperbolic and less steep in SHR, while the relationship for clonidine was linear in SHR but not in WKY. Alpha 1-adrenoceptor occupancy at a half-maximum response to each agonist was greater in SHR. Nifedipine inhibited the maximum responses to the agonists more profoundly in SHR than in WKY. This inhibition was greater in the response to clonidine than in the response to phenylephrine in both strains. When the maximum response to phenylephrine was reduced to the same extent in both strains by treatment with different concentrations of phenoxybenzamine, the responses to phenylephrine were more susceptible to inhibition by nifedipine. Under these conditions, the effects of nifedipine were similar between SHR and WKY. These results suggest that alpha 1-adrenoceptor reserve is reduced in SHR mesenteric artery compared with WKY, which may be responsible for the greater inhibition by nifedipine of the alpha 1-adrenoceptor-mediated contractions in SHR.