Na x is a brain Na + sensor expressed in the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT) in the brain. We previously demonstrated that Na x signals are involved in the control of water intake behavior through the Na x /TRPV4 pathway. Na x gene knockout mice showed significantly attenuated water intake after an intracerebroventricular (ICV) injection of a hypertonic NaCl solution; however, the induction of a certain amount of water intake still remained, suggesting that another unknown Na + -dependent pathway besides the Na x /TRPV4 pathway contributes to water intake. In the present study, we screened for novel Na + sensors involved in water intake control and identified SLC9A4 (also called sodium (Na + )/hydrogen (H + ) exchanger 4 (NHE4)). SLC9A4 is expressed in angiotensin II (Ang II) receptor type 1a (AT1a)-positive neurons in the OVLT. Sodium-imaging experiments using cultured cells transfected with slc9a4 revealed that SLC9A4 was activated by increases in extracellular Na + (Na + o ), but not osmolality. Moreover, the firing activity of SLC9A4-positive neurons was enhanced by increases in Na + o and Ang II. slc9a4 knockdown in the OVLT reduced water intake induced by increases in Na + , but not osmolality, in the cerebrospinal fluid. ICV injection experiments of a specific inhibitor suggested that the increase in extracellular H + caused by SLC9A4 activation next stimulates acid-sensing channel 1a (AS1C1a) to induce water intake. Our results thus indicate that SLC9A4 in the OVLT functions as a Na + sensor for the control of water intake and that the SLC9A4 signal is independent of the Na x /TRPV4 pathway.