Effects of nonsteroidal anti-inflammatory drugs (NSAID) on urine volume and urinary sodium excretion, and on plasma volume and extracellular fluid volume were examined in conscious rats. The basal urine volume and urinary sodium excretion were decreased and the increased urine volume and urinary sodium excretion elicited by saline load (25 ml/kg) and by hydrochlorothiazide (10 mg/kg) were inhibited after oral administration of NSAID in doses which inhibited the rat carrageenin-induced hind paw edema (indomethacin, 1--10 mg/kg; tolmetin, 3--30 mg/kg; phenylbutazone, 3--30 mg/kg; aspirin, 30--300 mg/kg), but aminopyrine (30--300 mg/kg) did not show such an effect. The inhibitory activity on renal function was diminished gradually with repeated administration of NSAID. NSAID (indomethacin, 3 mg/kg; tolmetin, 10 mg/kg; phenoxybenzamin, 10 mg/kg; aspirin, 100 mg/kg) increased plasma volume and extracellular fluid volume of rats after repeated medication for 3 or 5 days, but the body fluid volume expansion disappeared with further repeated administration of NSAID. These results suggest that NSAID may inhibit the intrarenal role of prostaglandins and decrease sodium and water excretion in urine with resulting increased body fluid volume. Tolerance to these actions of NSAID developed after repeated administration.