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The biological activity of synthetic polypeptides containing the amino acids of natural pure trypsin-bradykinin and snake-venom-bradykinin has been investigated. A series of tests for bradykinin-like activity in stimulating plain muscle, depressing the blood pressure and increasing capillary permeability was used on various species. A nonapeptide with the following structure: -Arg-OH also exerted bradykinin-like effects but was 50 to 100 times less active than the nonapeptide. Three other octapeptides and a heptapeptide were without any significant effect. Further work will demonstrate if the nonapeptide A is synthetic bradykinin or a peptide with bradykinin-like activity.Note added since submission of this paper: The data from this investigation were personally precommunicated to Elliott, Lewis, and Horton, who have since found that the structure of pure trypsin-bradykinin is identical with the structure of the nonapeptide A. Therefore, this synthetic nonapeptide is in fact synthetic bradykinin.Since the discovery of bradykinin, its polypeptide nature has been suggested and amply verified (Rocha e Silva, Beraldo and Rosenfeld, 1949; Rocha e Silva, 1955Silva, , 1960. The amino acid composition of bradykinin prepared by the action of trypsin on the fraction of ox plasma precipitating between 35 and 45% of saturation by ammonium sulphate (trypsin-bradykinin) was recently studied by Elliott, Lewis, and Horton (1960a) following its isolation; according to these investigators the molecule of trypsin-bradykinin contains two moles each of arginine, phenylalanine, and proline, and one mole each of TABLE I

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