The outer medullary collecting duct (OMCD) plays an important role in mediating transepithelial HCO transport () and urinary acidification. HCO absorption by type A intercalated cells in the OMCD inner stripe (OMCD_is) segment is thought to by mediated by an apical vacuolar H⁺-ATPase and H⁺-K⁺-ATPase coupled to a basolateral Cl-HCO exchanger (AE1). Besides these Na⁺-independent transporters, previous studies have shown that OMCD_is type A intercalated cells have an apical electroneutral EIPA-sensitive, DIDS-insensitive Na⁺-HCO cotransporter (NBC3); a basolateral Na⁺/H⁺ antiporter; and a basolateral Na⁺-K⁺-ATPase. In this study, we reexamined the Na⁺ dependence of transepithelial Na⁺ transport in the OMCD_is and determined the role of apical NBC3 in intracellular (pH_i) regulation in OMCD_is type A intercalated cells. Control tubules absorbed HCO at a rate of ~13 pmol · min¹ · mm¹. Lowering luminal Na⁺ from 140 to 40 mM decreased by ~15% without a change in transepithelial potential (V_te). Furthermore, 50 µM EIPA (lumen) also decreased by ~13% without a change in V_te. The effect of lowering luminal Na⁺ and adding EIPA were not additive. These results demonstrate that in the OMCD_is is in part Na⁺ dependent. In separate experiments, the pH_i recovery rate after an NH prepulse was monitored in single type A intercalated cells with confocal fluorescence microscopy. The pH_i recovery rate was ~0.21 pH/min in Na⁺-containing solutions and decreased to ~0.16 pH/min with EIPA (50 µM, lumen). In tubules perfused/bathed without Na⁺, luminal Na⁺ addition resulted in a pH_i recovery rate of ~0.36 pH/min, whereas the Na⁺-independent recovery rate was ~0.16 pH/min. EIPA (50 µM, lumen) decreased the Na⁺-dependent pH_i recovery rate to ~0.07 pH/min. The Na⁺-independent recovery rate was decreased to ~0.06 pH/min by bafilomycin (10 nM, lumen) and to ~0.10 pH/min using Schering 28080 (10 µM, lumen). These findings indicate that NBC3 contributes to pH_i regulation in OMCD_is type A intercalated cells and plays only a minor role in mediating in the OMCD_is.