Secretion of HCO3− at the apical side of the epithelial cells in the choroid plexus is an essential step in the formation of cerebrospinal fluid. Recently, Na+/HCO3− cotransporter 4 (NBC4), an electrogenic member of the NBC family, was identified in mouse and rat choroid plexus. The stoichiometry of NBC4 has been determined as 3 HCO3− : 1 Na+, which is favorable for the efflux of HCO3− and Na+ from the cell into CSF at the apical side (Millar and Brown, 2008). Very recently, we reported that a novel NBC4 isoform (NBC4g: truncated form of general isoform of NBC4c) expressed highly in rat choroid plexus and represented a DIDS-sensitive HCO3− transport activity (Fukuda et al.., 2013). In the present study, we further demonstrate that (1) RT-PCR analysis using specific primers of NBC4g and NBC4c indicated that NBC4g isoform was clearly expressed, but NBC4c isoform was not detected in the rat choroid plexus. (2) Immunostaining and electron microscopy studies revealed the apical localization of NBC4g in the choroid epithelial cells. (3) Electrophysiological studies using Xenopus oocytes injected with NBC4g cRNA showed a forskolin, an activator of membrane adenylyl cyclase (mAC),-dependent outward current in the presence of 5% CO2/33 mM HCO3− in the bathing medium. These results support the hypothesis that NBC4g activity may be regulated by cAMP-dependent protein kinase (PKA) phosphorylation at one or more of the three PKA predicted sites. In conclusion, NBC4g, densely localized to the apical surface of the choroid epithelial cells, may contribute to the production of CSF and regulate its volume and ionic composition in a cAMP-dependent manner.