SUMMARY: A method for the bioassay of the pressor substances in extracts of the urophyses of the teleost fish, Mugil auratus and Anguilla anguilla, is described. Ultracentrifugation, chromatography on Sephadex gel and treatment with trypsin and papain showed that the pressor principles are polypeptides. One fraction was protein-bound while two other fractions seem to be small molecules. Treatment with thioglycollate showed that disulphide bonds do not form an integral part of the active molecule. The possible function of these principles in the control of the caudal circulation is discussed.