The effects on the renal actions of bumetanide of two inhibitors of prostaglandin synthetase, acetylsalicylic acid (ASA) and indomethacin, were studied in eight normal adults. Indomethacin alone resulted in a significant decrease in FENa compared to the control period. Bumetanide alone resulted in increases in urine volume, FENa, and plasma renin activity. Both ASA and indomethacin blunted the increases in urine volume and sodium excretion, although the inhibition by indomethacin was greater than that of ASA. In addition, indomethacin completely inhibited the bumetanide-induced increase in plasma renin activity. No consistent relationship between these effects and urinary prostaglandin E2 or F excretion was noted. These studies indicate that prostaglandin synthetase inhibitors may interfere with the effects of bumetanide.