It is believed that the renin-angiotensin system evolved initially in primitive bony fishes and is absent from elasmobranchs. We have isolated angiotensin I from the incubates of plasma and kidney extracts of an elasmobranch fish, Triakis scyllia, using eel vasopressor activity as an assay system. Its sequence was determined to be H-Asn-Arg-Pro-Tyr-Ile-His-Pro-Phe-Gln-Leu-OH. Dogfish angiotensin I is teleost-like because of an asparagine residue at position 1 but it is mammalian-like because of an isoleucine residue at position 5. The unique and most important substitution in dogfish angiotensin I is a proline residue at position 3 which may cause significant changes in its tertiary structure. A glutamine residue at position 9 is also unique among all angiotensin Is sequenced to date. Dogfish angiotensin I is more potent than rat angiotensin I in its vasopressor activity in the dogfish but the relationship is reversed in the rat. Thus angiotensin receptors as well as the hormone molecules appear to have evolved during vertebrate phylogeny. Our findings establish the elasmobranch renin-angiotensin system and support the hypothesis that the renin-angiotensin system is a phylogenetically old hormonal system which plays important roles in cardiovascular and fluid homeostasis.