Structurally different inhibitors of prostaglandin synthesis reversed the antagonistic activity of the angiotensin II analog 1-sarcosine, 8-threonine angiotensin II. In isolated rabbit aorta, indomethacin or ETYA (eicosa 5, 8, 11, 14, tetraynoic) reversed the antagonism of the angiotensin II analog without affecting the dose response curves to angiotensin II. Both indomethacin and ETYA were able to reverse the tachyphylaxis to angiotensin II. Investigating the possibility that indomethacin could affect the binding of angiotensin II, it was found that at doses of indomethacin higher than 10<sup>–5</sup> m, binding of tritiated angiotensin to isolated adrenal cortical cells was reduced. It is postulated that indomethacin may have more than one effect on angiotensin, possessing a concentration-dependent effect on binding and prostaglandin synthesis.