The biliary excretion of vanadium was studied in bile duct cannulated rats at various times after the iv administration of a single dose of 48V-labelled pentavalent vanadium. The tissue disposition, hepatic intracellular distribution and binding of 48V to plasma, bile and liver components were also investigated. During the first 6 hours after the injection of V, 0.9 to 30 microgram/kg, less than 2 per cent of the dose was excreted into the bile and up to 20 per cent of vanadium was eliminated in urine. The bile flow was markedly decreased in rats cannulated 24 hours after the injection of V, 30 microgram/kg, while higher doses produced signs of severe toxicity immediately after the administration. Evidence was also obtained that a concentration gradient triggers the passage of vanadium from plasma to the liver, where only a fractional amount becomes available from canalicular excretion, due to the extensive binding of the metal to organelles and other tissue ligands.