The kinin peptide system in humans is complex. Whereas plasma kallikrein generates bradykinin peptides, glandular kallikrein generates kallidin peptides. Moreover, a proportion of kinin peptides is hydroxylated on proline³ of the bradykinin sequence. We established HPLC-based radioimmunoassays for nonhydroxylated and hydroxylated bradykinin and kallidin peptides and their metabolites in blood and urine. Both nonhydroxylated and hydroxylated bradykinin and kallidin peptides were identified in human blood and urine, although the levels in blood were often below the assay detection limit. Whereas kallidin peptides were more abundant than bradykinin peptides in urine, bradykinin peptides were more abundant in blood. Bradykinin and kallidin peptide levels were higher in venous than arterial blood. Angiotensin-converting enzyme inhibition increased blood levels of bradykinin, but not kallidin, peptides. Reactive hyperemia had no effect on antecubital venous levels of bradykinin or kallidin peptide levels. These studies demonstrate differential regulation of the bradykinin and kallidin peptide systems, and indicate that blood levels of bradykinin peptides are more responsive to angiotensin-converting enzyme inhibition than blood levels of kallidin peptides.