An overview is given of the absorption, distribution, excretion, and in vivo metabolism of drugs in fish. Several methods for chemical disposition and metabolism in fish are outlined, and a few well-characterized studies of the relationships between xenobiotic metabolism and toxicity in aquatic species are highlighted. For more than 40 drugs and other xenobiotics in several species of fish, concentrations of chemicals in bile were usually considerably higher than values in other body compartments or in the surrounding water. This was shown to be true particularly for parent substances or their biotransformed products with molecular weights greater than 400. The use of bile analysis is recommended as a regulatory tool to detect contamination of fish from drug treatment or environmental exposure.