The integrated response to hypotonic NaCl-solutions (100, 50, 25 and 0 mM NaCl) in proximal duodenum of anesthetized rats was examined. Luminal alkalinization, fluid flux, duodenal contractions, blood-to-lumen clearance of ⁵¹Cr-EDTA (mucosal permeability) and perfusate osmolality were studied in the absence and presence of the COX inhibitor indomethacin. In response to hypotonic solutions net fluid absorption, increases in permeability and perfusate osmolality were markedly higher in indomethacin-treated animals than in controls and these effects were diminished by the nicotinic receptor antagonist hexamethonium. Infusion of iloprost, a stable PGI₂-analogue, to indomethacin-treated animals markedly reduced the hypotonicity-induced increase in mucosal permeability and diminished the rise in perfusate osmolality. Lowering the NaCl concentration in the perfusion solution, but maintaining isotonicity with mannitol, had no effect on mucosal permeability. Very good linear correlations were obtained between the degree of luminal hypotonicity and the increase in permeability and between increases in permeability and perfusate osmolality. It is concluded that luminal hypotonicity increases duodenal mucosal permeability. The hypotonicity-induced increase in permeability, modulated by prostaglandins and nicotinic receptors, fulfills the function of increasing blood-to-lumen transport of Na⁺ facilitating adjustment of luminal osmolality.