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Some species of lampreys are anadromous migrants between (FW) and seawater (SW) and adjust ionic and osmoregulatory mechanisms in a similar way to teleost fish (Rankin, 1997). Hyper-osmoregulation in FW demands excretion of large volumes of urine, whilst hypo-osmotic regulation in SW depends on increasing drinking and drastically reducing urine output. Endocrine control of body fluid volume and composition in lampreys is poorly understood and existence of a lamprey renin-angiotensin system (RAS) has until recently been in doubt (Henderson et al., 1993). This endocrine system plays a key role in regulation of cardiovascular function and blood volume homeostasis in many vertebrates and is involved in survival of teleost fish in high external salinities. Recent isolation and sequencing of endogenous angiotensin I (Ang I) in Lampetra fluviatilis and Petromyzon marinus (Rankin, Watanabe, Nakajima and Takei, unpublished) led to our measurement of the physiologically active angiotensins (angiotensin II and III: Ang II & III) (Rankin et a