NHE3 is one of five plasma membrane Na+/H+ exchangers 1, 2, 3 and is encoded by the mouse gene Slc9a3. It is expressed on apical membranes of renal proximal tubule 4, 5 and intestinal epithelial cells 6, 7 and is thought to play a major role in NaCl and HCO 3–absorption 4, 5, 6, 7, 8, 9, 10. As the distribution of NHE3 overlaps with that of the NHE2 isoform in kidney 7, 11 and intestine 7, 12, 13, the function and relative importance of NHE3 in vivo is unclear. To analyse its physiological functions, we generated mice lacking NHE3 function. Homozygous mutant (Slc9a3–/–) mice survive, but they have slight diarrhoea and blood analysis revealed that they are mildly acidotic. HCO 3–and fluid absorption are sharply reduced in proximal convoluted tubules, blood pressure is reduced and there is a severe absorptive defect in the intestine. Thus, compensatory mechanisms must limit gross perturbations of electrolyte and acid-base balance. Plasma aldosterone is increased in NHE3-deficient mice, and expression of both renin and the AE1 (Slc4a1) Cl–/HCO 3–exchanger mRNAs are induced in kidney. In the colon, epithelial Na+ channel activity is increased and colonic H+, K+-ATPase mRNA is massively induced. These data show that NHE3 is the major absorptive Na+/H+ exchanger in kidney and intestine, and that lack of the exchanger impairs acid-base balance and Na+-fluid volume homeostasis.