The effect of extracellular and intracellular pH on vascular tone and ⁴⁵Ca uptake were investigated in aortic strips and dispersed single aortic smooth muscle cells of spontaneously hypertensive rats (SHR) and aged-matched Wistar-Kyoto rats (WKY). The contraction produced by a change of extracellular pH (pHo) in the range of 6.5 ~ 8.3 was estimated by comparison with the level of vascular tone at pH 7.4. Contraction was induced below pHo 6.5 in WKY, pHo 7.1 in SHR, and over pHo 8.0 on both strains. The amplitude of contraction induced by high pHo (over pHo 7.7) was similar in SHR and WKY, but that induced by low pHo (below pHo 7.1) in SHR was greater than that in WKY. Either high pHo- or low pHo-induced contractions in WKY and SHR were not induced in the Ca-free Tyrode s solution and were induced by the addition of Ca. ⁴⁵Ca uptake increased progressively as pHo was increased from 6.8 to 8.1 in the single aortic smooth muscle cells of WKY and SHR. NH₄Cl induced a gradually developing contraction in a dose-dependent manner (5 mM ~ 30 mM) and the removal of NH₄Cl induced transient contraction was followed by profound relaxation in the aortic rings of both strains. The contractions induced by NH₄Cl or by the removal of NH₄Cl in SHR were significantly greater than that in WKY. These contractions were not induced in Ca-free Tyrode s solution. ⁴⁵Ca uptake was increased by NH₄Cl (20 mM) and was not changed by the removal of NH₄Cl (20 mM) in the aortic strips of WKY and SHR. As a summary of above results, the vascular tone of SHR was more sensitive to the change pHi and pHo than that of WKY. The contractions induced by change of extracellular or intracellular pH depended on extracellular Ca in the aorta of SHR nnd WKY. However, the Ca uptake was in accord with the changes of contraction but increase in contraction by low pH was not accompanied by an increase in Ca uptake in both strains.