Background: Urinary acidification and polyuria regulated by stimulation of calcium sensing receptor (CaSR) may reduce the risk for urolithiasis caused by hypercalciuria (Renkema KY et al. 2009). However, expression level of CaSR, localized at the basolateral membrane of intercalated cell type B (IC-B) in the mouse kidney collecting duct (CD), was unexpectedly decreased during metabolic acidosis caused by NH4Cl-loading (Yasuoka Y et al. 2011). Thus, we further investigated a role of the CaSR for the Ca and pH metabolism in the kidney using a combination of functional and molecular biology technique. Methods: C57BL/6J mice (male, 10 weeks) were subjected to (1) normal (1%) Ca diet (control, n=6), (2) high (2.5%) Ca diet (calcium (CaX) loading, n=6), and (3) NH4Cl-loading (2.5%) diet (acidosis, n=6). For blood analysis, heparinized blood, collected from the carotid artery of mice anesthetized with 1.5% isoflurane, was analyzed as usual. Values are means ± SE., compared by Student’s t-test. Results: The level of blood [Ca2+] remained in normal ranges even after administration of CaX and NH4Cl for 28 and 6 days, respectively. Blood pH of CaX-loading mice was also unchanged (7.37 ± 0.02), whereas that of NH4Cl-loading mice significantly decreased (7.25 ± 0.02, P < 0.005) compared with control (7.38 ± 0.01). Urinary [Ca2+] of CaX- and NH4Cl-loading mice was significantly increased (6.2 ± 0.1 mg/dl (P < 0.05) and 12.1 ± 0.9 mg/dl (P < 0.005), respectively), compared with control (5.6 ± 0.4 mg/dl). Urinary pH of CaX- and NH4Cl-loading mice was significantly lower (6.07 ± 0.07 (P < 0.005) and 5.96 ± 0.03 (P < 0.005), respectively) than control (6.48 ± 0.04). When mice were kept with NH4Cl for 6 days, the expression of H-ATPase, carbonic anhydrase (CA) XII and anion exchanger (AE) 1 increased, whereas the levels of the transient receptor potential vanilloid 5 (TRPV5), CaSR, and pendrin expression were decreased. On the other hands, the expression levels of all these molecules were markedly increased by CaX-loading. Interestingly, NH4Cl-loading increased cell height of IC-A in CDs and CaX-loading increased it at both IC-A and IC-B. All these results suggest that hypercalciuria, reduced reabsorption of Ca2+ due to acidosis-induced downregulation of TRPV5 at the distal convoluted tubule (DCT), may stimulate acid secretion of IC-A and alkali secretion of IC-B in parallel, probably due to an activation of CaSR of IC-B through unknown mechanisms. Conclusions: CaSR of IC-B may contribute to alkali secretion to prevent urolithiasis in the mouse kidney collecting duct during dietary surplus Ca loading.