A protecting role of selenite against the toxic effects of different mercury compounds is well documented. The organ distribution and retention of inorganic mercury as well as of methylmercury seem to be influenced by a high selenium status. Mercury is characteristically present in the aquatic environment and follows the food chain. Several authors have shown a correlation between high concentrations of mercury and selenium in aquatic organisms. Virtually nothing is known about the interaction of methylmercury and selenium in feeding experiments on fish. Therefore the present study was designed to determine the possible interaction of methylmercury and selenium in different tissues of cod (Gadus morhua) by means of feeding diets supplemented with nontoxic levels of the elements.