Numerous xenobiotics are found in anthropogenically altered aquatic environments. These chemicals often are available for uptake by aquatic organisms. One of the important issues in aquatic toxicology is our ability to predict the rate and extent of organismic uptake of pollutant chemicals as well as the in vivo fate and effects of the absorbed chemicals. It has been well documented that animals living in chemically polluted aquatic environments will acquire body burdens of the chemical pollutants. I-3 The actual body burden acquired by an individual animal will depend on a number of factors, including the physicochemical properties of the xenobiotic chemical, the routes of exposure, and the physiological and biochemical make-up of the animal. Several routes of exposure are possible. Chemicals dissolved or suspended in water may enter through the gills, the skin, or the gastrointestinal tract. Chemicals present in sediment may be absorbed by direct dermal contact and ingestion, while chemicals present in plants and low trophic-level organisms may be ingested and absorbed through the gastrointestinal tract. The relative importance of each route of entry varies with the intrinsic chemical properties of each xenobiotic and also with the likelihood that the chemical under consideration will be degraded in the aquatic environment or biotransformed by low trophic-level organisms. There is still much to be learned about the impact of preconsumptive biotransformation by

plants and animals on the fate of contaminants in the consumer species and about the relative importance of dietary pollutants to the overall body burdens of aquatic species. Knowledge of the bioavailability and biotransformation of chemical contaminants in components ofthe aquatic food chain may be of aid in defining the risk to animals and humans of consumption of fish and shellfish from polluted waters.