The mechanisms by which endogenous renal prostaglandins regulate water excretion were investigated in these studies. Inhibition of prostaglandin synthesis slowed water diuresis in water-loaded unanesthetized Sprague-Dawley rats and in Brattleboro rats with hereditary diabetes insipidus owing to absence of endogenous vasopressin. In both strains, treatment with indomethacin or meclofenamate increased the osmolality of the renal papilla by raising sodium and urea content, and also increased the osmolality of the urine. Endogenous creatinine clearance and solute excretion were unchanged. The data are consistent with an effect of prostaglandins on solute transport by renal tubules and demonstrate that endogenous prostaglandins influence water excretion by a mechanism independent of the presence of antidiuretic hormone.