Vanadium is present in natural waters predominantly as the orthovanadate anion V03/4- in concentrations of about 10⁻⁸M. Orthovanadate perfused through the gills of fresh water or sea water adapted eels induces vasoconstriction at concentrations as low as 5 x 10⁻⁸M. The pillar cells of the respiratory lamellae and the afferent and efferent blood vessels of the gill filament are affected by orthovanadate. If orthovanadate 5 x 10⁻⁸M is perfused through the gills of sea water adapted eels, the outfluxes of Na+ and Cl- are significantly reduced. Vanadium is taken up by young eels (elvers) in a manner dependent on the concentration of orthovanadate in the environment over the range 10⁻⁸-10⁻⁵M. Vanadium is deposited in the kidney, liver, bone, blood and skeletal muscle. An environmental concentration of 10⁻⁴M orthovanadate is rapidly toxic to elvers causing death within 7-26 days of exposure. Elvers loaded with vanadium through 2 months exposure to 10⁻⁵M orthovanadate show little sign of depleting their vanadium burden after 5 weeks exposure to 10⁻⁸M orthovanadate. The eel intestine is the main site of entry of vanadium into the eel, gill contributes <5% of the total uptake of orthovanadate by the eel. The influx of orthovanadate across eel intestine is dependent on the concentration of orthovanadate in the external environment. The results are discussed in relation to the published literature on the physiological and biochemical actions of vanadium and the extent to which vanadium might be an environmental hazard is assessed.