We compared the branches and trunk of rat superior mesenteric artery (SMA) with respect to extracellular pH (pH_o)-dependent changes in vascular contractility. Decreases in pH_o from 7.8 to 6.4 significantly reduced pD₂ to norepinephrine (NE) and maximal contraction by NE, which were more prominent in larger diameter artery. On the other hand, decreases in pH_o significantly reduced Ba²⁺-sensitive K⁺-induced relaxation, which was evoked by elevation of extracellular K⁺ concentration from 6 mM to 12 mM, in the first branch and inhibited inwardly rectifying K⁺ (K_ir) currents in cultured smooth muscle cells (SMCs) of SMA. RT-PCR revealed transcripts for K_ir2.1 in the SMCs. Real time PCR analysis revealed 6.1-, 3.3-, and 2.2-fold increases in the K_ir2.1 mRNA to -actin mRNA ratios of SMCs of the third, second and first branches, respectively, versus the corresponding relative levels of trunk SMCs. The magnitudes of K⁺-induced relaxation were significantly greater in smaller diameter arteries and there was a strong correlation between the transcript levels of K_ir2.1 and K⁺-induced relaxation. A decrease in pH_o reduced ouabain-sensitive K⁺-induced relaxation and ouabain-induced contraction. A decrease in pH_o from 7.4 to 6.4 depolarized membrane potential of the cultured SMCs. From these results, we conclude that an increase in pH_o activates K_ir currents and the Na⁺-K⁺ pump, which then reduces vascular contractility. Inasmuch as K_ir channel densities are significantly greater in smaller diameter arteries, the reduction in vascular contractility on increasing pH_o is more pronounced in smaller diameter arteries.