S tudies were performed to determine the effect of decreased endogenous release of renal prostaglandins on urinary sodium excretion. Two structurally dissimilar inhibitors of prostaglandin synthesis were employed, and studies were performed in conscious dogs allowed to recover from prior surgical instrumentation. Either meclofenamate (2 mg/kg) or the competitive prostaglandin inhibitor RO 20-5720 (1 mg/kg) was given to seven unanesthetized dogs undergoing a water diuresis. The administration of either prostaglandin inhibitor did not alter glomerular filtration rate, renal plasma flow, urinary volume, or potassium excretion. Sodium excretion, however, increased from 32 to 130 mueq/min (P less than 0.02). Essentially, the entire increase in sodium excretion was due to an increase in urinary sodium concentration from 7.7 to 28.3 meq/liter (P less than 0.02). On a different day, the same animals were studied before and after administration of the diluent of the prostaglandin inhibitor. No change was noted in sodium excretion or any other parameter. Thus, these findings suggest that prostaglandin inhibition in the conscious dog is associated with a natriuresis without a change in urinary volume or potassium excretion during water diuresis. This may indicate that the natruiresis was due to diminished sodium reabsorption beyond the distal tubule.