The outer medullary collecting duct (OMCD) plays an important role in mediating transepithelial bicarbonate transport and urinary acidification. Bicarbonate absorption by type A intercalated cells in the OMCDis (inner stripe) segment is thought to by mediated by an apical vacuolar H⁺-ATPase and H⁺-K⁺-ATPase coupled to a basolateral Cl^--HCO₃^- exchanger (AE1). In addition to these Na⁺-independent transporters, previous studies have shown that OMCDis type A intercalated cells have an apical electroneutral EIPA-sensitive DIDS-insensitive sodium bicarbonate cotransporter (NBC3), a basolateral Na⁺/H⁺ antiporter, and a basolateral Na⁺-K⁺-ATPase. In this study we re-examined the Na⁺-dependence of transepithelial Na⁺ transport in the OMCDis. In addition, we determined the role of apical NBC3 in pH_i regulation in OMCDis type A intercalated cells. Control tubules absorbed bicarbonate at a rate of ~ 13 pmol/min/mm. Lowering luminal Na⁺ from 140 mM to 40 mM decreased JHCO₃^- by ~15% without a change in transepithelial potential (Vte). Furthermore, 50 µM EIPA (lumen) also decreased JHCO₃^- by ~13% without a change in Vte. The effect of lowering luminal Na⁺ and EIPA addition were not additive. These results demonstrate that transepithelial bicarbonate transport in the OMCDis is in part sodium dependent. In separate experiments, the pH pH_i recovery rate following an NH₄⁺ prepulse was monitored in single type A intercalated cells with confocal fluorescence microscopy. The pHi recovery rate was ~ 0.21 pH/min in Na⁺-containing solutions and decreased to ~ 0.15 pH/min with EIPA (50 µM, lumen). In tubules perfused/bathed without Na⁺, luminal Na⁺ addition resulted in a pHi 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 pHi 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 OMCDis type A intercalated cells, and plays only a minor role in mediating transepithelial bicarbonate transport in the OMCDis.