1. In the isolated rat kidney, perfused at constant pressure with a medium free from renin substrate, addition of angiotensin I to the perfusate decreases renal ‘plasma’ flow.
2. A peptide inhibitor, SQ 20881, of converting enzyme reduces the vasoconstrictor effect of angiotensin I up to a maximum of 87%, the degree of inhibition being dose-dependent.
3. The molar, ratio of equi-effective doses of angiotensin I and angiotensin II was 50:1, indicating a low rate of intrarenal conversion of the decapeptide.
4. The vasoconstrictor effect elicited by the addition of renin substrate to the perfusate was not inhibited by SQ 20881, even if the concentration was fifteen times that which produced the maximum inhibition of conversion of angiotensin I.