Recent advances in urophysial research are reported and current research in this laboratory on the urophysial peptides (urotensin I, urotensin II) of Catostomxts commersoni is summarized. Urotensin I has been purified by gel filtration, electrofocussing, ultrafiltration and ion exchange chromatography. Three similar utotensin I peptides were found. The main peptide was shown to be a straight chain peptide with about 36 amino acid residues. Pressor-diuretic effects in fish have been observed. The peptide produces a long-lasting lowering of mammalian blood pressure by peripheral vasodilatation. Urotensin II has been isolated and purified. It appears to contain 23 amino acids including two disulphidebridges. It is a potent pressor-diuretic-smooth muscle contracting agent in the fish, without any known pharmacological actions in mammals. Urotensin III: the existence of this activity as a separate chemical entity has not been established, but urophysectomy was shown to lower blood Na+ without an increase in renal excretion. Urotensin IV has been shown to be similar to, or identical with, arginine vasotocin. Urotensin IV could not be found in the urophysis of Catostomus (gel filtration, bioassays, immunohistochemistry). Similarities and differences between the hypothalamic and caudal neurosecretory systems and their active principles are analysed.