The hepatic metabolism of 35S-sulfobromophthalein was studied in the free-swimming Elasmobranchs, Squalus acanthias (spiny dogfish shark) and Raja erinacea (small skate), to evaluate the physiological significance of biliary excretion of organic anions in marine vertebrates. Twenty-four hours after intra-arterial injection, 85.8 +/- 15.7% of the administered dose was recovered in bile and liver in dogfish and 78.4 +/- 9.9% in skates. More than 85% of BSP in bile was in unconjugated form in both species. Furthermore, selective hepatic uptake and biliary excretion of 35S-sulfobromophthalein occurred despite very little specific binding protein (Ligandin). These studies demonstrate that transport systems for biliary excretion of organic anions evolved before migration of marine life from the sea and independent of conjugation and organic anion binding proteins which facilitate biliary excretion of compounds such as 35S-sulfobromophthalein in higher vertebrates.