We examined the effect of norepinephrine (NE) on intracellular pH (pH_i) and activity of Na⁺ (a^Na_i) in the isolated perfused kidney proximal tubule of Ambystoma, using single-barreled voltage and ion-selective microelectrodes. In control HCO₃ Ringer, addition of 10⁶ M NE to the bath reversibly depolarized the basolateral membrane potential (V₁), the luminal membrane potential (V₂), and the transepithelial potential difference (V₃) and increased pH_i by 0.14 ± 0.02. These effects were mimicked by isoproterenol but were abolished after pretreatment with SITS or in the absence of CO₂/HCO₃. Removal of bath Na⁺ depolarized V₁ and V₂, hyperpolarized V₃, and decreased pH_i. These effects are largely mediated by the electrogenic Na⁺-(HCO₃)_n cotransporter. In the presence of NE, the effects of Na⁺ removal on membrane potential differences and the rate of change of pH_i were significantly smaller. Reducing bath HCO₃ concentration from 10 to 2 mM at constant CO₂ (pH 6.8) depolarized V₁ and V₂, decreased pH_i, and lowered a^Na_i. These changes are also due to Na⁺-(HCO₃)_n. In the presence of NE, reducing bath [HCO₃] caused a smaller depolarizations of V₁ and V₂, and the rate of pH_i decrease was significantly reduced. Our results indicate: 1) NE causes an increase in pH_i; 2) the NE-induced alkalinization is mediated by a SITS-sensitive and HCO₃-dependent transporter on the basolateral membrane; and 3) in the presence of NE, the reduced effects caused by basolateral HCO₃ changes or Na⁺ removal are indicative of an inhibitory effect of NE on Na⁺-(HCO₃)_n cotransport.