Degradation of bradykinin (BK) in human plasma was investigated by searching for a stable metabolite as a marker of kinin release in vivo. BK, incubated with diluted plasma, was degraded to des-Arg9-BK (des-9-BK), des-Phe8-Arg9-BK (des-8,9-BK) and Arg-Pro-Pro-Gly-Phe ([1-5]BK). Des-9-BK was degraded to [1-5]BK without an increase in des-8,9-BK, and des-8,9-BK was also degraded to [1-5]BK. D,L-2-Mercaptomethyl-3-guanidinoethyl-thiopropanoic acid inhibited the formation of des-9-BK from BK, whereas captopril inhibited the formation of des-8,9-BK from BK as well as that of [1-5]BK from des-9-BK or des-8,9-BK. The half lives of BK, des-9-BK, des-8,9-BK and [1-5]BK under the present experimental conditions were 60 min, 90 min, 14 min and 4.2 hr, respectively. Among the metabolites, [1-5]BK was the most stable one and may be used as a marker for BK production in vivo.