Studies were conducted to determine whether or not angiotensin III [AIII] and angiotensin II-(3-8)-hexapeptide [ANG-(3-8)] have their own specific arteriolar binding sites different from angiotensin II [AII] binding site(s) in man. Four patients with Bartter's syndrome were given asn1-,val5-AII by iv infusion at rates of 10, 20, 50 and 100 pmol/kg X min, each for 7 min. One hour later AIII was infused iv in the same 4 patients at rates of 50, 100, 250 and 500 pmol/kg X min, each for 7 min. After 100 or 150 mg/day of indomethacin treatment for 7 days, the same AII and AIII infusions were repeated. All patients showed blunted pressor responses to both AII and AIII before indomethacin and the responses were improved after indomethacin. Moreover, increment curves of blood pressure for AII were almost identical with those for AIII in individual patients both before and after indomethacin. ANG-(3-8) was infused iv in 3 normal men and 3 of the 4 patients with Bartter's syndrome at a rate of 3.500 pmol (2.838 ng)/kg X min for 15 min. Blood pressure rose in the normal men (12/12 mmHg on the average) but did not rise in the patients. These results suggest that AII, AIII and ANG-(3-8) have the same arteriolar binding sites in man.