The calcium-sensing receptor (CaSR) is activated by extracellular calcium (Ca) and mediates many of the known effects of extracellular divalent minerals on body cells. Both surface and crypt cells express CaSR transcripts and protein on both apical and basolateral surfaces. Raising Ca elicited increases in intracellular calcium (Ca) in both surface and crypt cells with an EC₅₀ of 2 mM. The Ca-induced increase in Ca was associated with increases in inositol 1,4,5-trisphosphate and eliminated by U-73129, an inhibitor of phosphatidylinositol-phospholipase C, as well as by thapsigargin. Other CaSR agonists, Gd³⁺ and neomycin, mimicked these Ca-induced responses. Both luminal and bath Ca, Gd³⁺, and neomycin induced increases in Ca in isolated perfused crypts. The stimulatory effect of forskolin on net fluid secretion in perfused crypts was abolished by increasing Ca in either luminal or bath perfusates. Thus both apical and basolateral CaSR on crypt cells are functional and provide pathways modulating net intestinal fluid transport that may have important implications for the prevention and treatment of certain diarrheal diseases associated with elevated cAMP.