The degree to which loss of the NHE3 Na(+)/H(+) exchanger in the kidney contributes to impaired Na(+)-fluid volume homeostasis in NHE3-deficient (Nhe3(-/-)) mice is unclear because of the coexisting intestinal absorptive defect. To more accurately assess the renal effects of NHE3 ablation, we developed a mouse with transgenic expression of rat NHE3 in the intestine and crossed it with Nhe3(-/-) mice. Transgenic Nhe3(-/-) (tgNhe3(-/-)) mice tolerated dietary NaCl depletion better than nontransgenic knockouts and showed no evidence of renal salt wasting. Unlike nontransgenic Nhe3(-/-) mice, tgNhe3(-/-) mice tolerated a 5% NaCl diet. When fed a 5% NaCl diet, tgNhe3(-/-) mice had lower serum aldosterone than tgNhe3(-/-) mice on a 1% NaCl diet, indicating improved extracellular fluid volume status. Na(+)-loaded tgNhe3(-/-) mice had sharply increased urinary Na(+) excretion, reflective of increased absorption of Na(+) in the small intestine; nevertheless, they remained hypotensive, and renal studies showed a reduction in glomerular filtration rate (GFR) similar to that observed in nontransgenic Nhe3(-/-) mice. These data show that reduced GFR, rather than being secondary to systemic hypovolemia, is a major renal compensatory mechanism for the loss of NHE3 and indicate that loss of NHE3 in the kidney alters the set point for Na(+)-fluid volume homeostasis.