A high-NaCl diet induces renal medullary cyclooxygenase (COX)2 expression, and selective intramedullary infusion of a COX2 inhibitor increases blood pressure in rats on a high-salt diet. The present study characterized the specific prostanoid contributing to the antihypertensive effect of COX2. C57BL/6J mice placed on a high-NaCl diet exhibited increased medullary COX2 and microsomal prostaglandin E synthase1 (mPGES1) expression as determined by immunoblot and real-time PCR. Cytosolic prostaglandin E synthase and prostacyclin synthase were not induced by the high-salt diet. Immunofluorescence showed mPGES1 in collecting ducts and interstitial cells. High salt increased renal medullary PGE₂as determined by gas chromatography/negative ion chemical ionization mass spectrometry. The effect of direct intramedullary PGE₂infusion was examined in anesthetized uninephrectomized mice. Intramedullary PGE₂infusion (10 ng/h) increased urine volume (from 3.3 ± 0.6 to 9.5 ± 1.6 μl/min) and urine sodium excretion (0.11 ± 0.02 to 0.32 ± 0.05 μeq/min). To determine which E-prostanoid (EP) receptor(s) mediated PGE₂- dependent natriuresis, EP-selective prostanoids were infused. The EP₂agonist butaprost produced natriuresis (from 0.06 ± 0.02 to 0.32 ± 0.05 μeq/min). The natriuretic effect of intramedullary PGE₂or butaprost was abolished in EP2-deficient mice, which exhibit NaCl-dependent hypertension. In conclusion, a high-salt diet increases renal medullary COX2 and mPGES1 expression, and increases renal medullary PGE₂synthesis. Renal medullary PGE₂promotes renal sodium excretion via the EP2 receptor, thereby maintaining normotension in the setting of high salt intake.