Bradykinin (2-9) is an amino-truncated Bradykinin peptide. Bradykinin (2-9) is a metabolite of Bradykinin, cleaved by Aminopeptidase P. In Vitro: The Bradykinin peptide system is a tissue-based system with potent cardiovascular and renal effects. To investigate the regulation of this system, a highly sensitive amino terminal-directed radioimmunoassay that, with high performance liquid chromatography, enables the measurement of Bradykinin-(1-7), Bradykinin-(1-8), and Bradykinin-(1-9), is developed. Together with a carboxy terminal-directed radioimmunoassay, Bradykinin peptides in rat kidney and blood are characterized. The predominant Bradykinin peptides in kidney are Bradykinin-(1-9) (~100 fmol/g wet weight of tissue) and Bradykinin-(1-7) (~70 fmol/g), with low levels of Bradykinin-(1-8) (~8 fmol/g) and Bradykinin-(4-9) (~12 fmol/g) detectable; Bradykinin-(2-9) and Bradykinin-(3-9) are below the limits of detection. In blood, the levels of Bradykinin-(1-9) are very low (~2 fmol/ml), and other Bradykinin peptides are below the limits ofdetection. Administration of the angiotensin converting enzyme (ACE) inhibitor Perindopril is associated with an approximate twofold increase in renal levels of Bradykinin-(1-8) and Bradykinin-(1-9) and a decrease in the Bradykinin-(1-7)/Bradykinin-(1-9) ratio. The amino terminal-directed radioimmunoassay is also applied to heart, aorta, brown adipose tissue, adrenal, lung, and brain. For these tissues, Bradykinin-(1-7) and Bradykinin-(1-9) are of similar abundance (16-340 fmol/g), with lower levels of Bradykinin-(1-8)^[1].