Bradykinin was converted rapidly into the fragment 1-8 by kininase I when incubated with human plasma. Kininase II inactivated bradykinin by splitting the Pro⁷-Phe⁸ and the Phe⁵-Ser⁶ bonds in it. ACE inhibitory peptide Val-Tyr and captopril did not inhibit the conversion of bradykinin in human plasma into the fragments 1-8, 1-7 and 1-5. Bradykinin was converted into the fragments 1-7 and 1-5 by dipeptidyl peptidase activity of ACE. Captopril inhibited degradation of bradykinin by dipeptidyl peptidase activity of ACE, but Val-Tyr did not inhibit.