Bradykinin is a short-lived vasoactive peptide, with a reported half-life in vivo of 17 s, that is rapidly metabolized in the circulation to Bradykinin (1-5). Bradykinin (1-5), the product of two sequential cleavages of Bradykinin by ACE at the Pro7-Phe8 and Phe5-Ser6bonds, has been identified as the major stable metabolite of Bradykinin in vivo in human subjects, with a terminal half-life of minutes. Both Bradykinin and Bradykinin (1-5) inhibit α- and γ-thrombin-induced platelet aggregation (P<0.01 versus baseline). Bradykinin (1-5) inhibits γ-thrombin-induced platelet aggregation 50% at a calculated dose of 183±3 pmol/min. Neither Bradykinin nor Bradykinin (1-5) affects thrombin receptor-activating peptide-induced platelet aggregation, consistent with the hypothesis that Bradykinin and Bradykinin 1-5 inhibit thrombin-induced platelet aggregation by preventing cleavage of the thrombin receptor and liberation of thrombin receptor-activating peptide. Bradykinin (1-5) significantly attenuates α-thrombin-induced platelet aggregation but not TRAP 1-6-induced platelet aggregation. Bradykinin (1-5) potently inhibits γ-thrombin (500 nM)-induced platelet aggregation with an ED ₅₀ of 183±2 pmol/min.