This paper describes the tissue distribution of inorganic mercury, organic mercury selenium in Dutch and British seals as well as in seals which were dosed with methylmercury under experimental conditions. In the experimental animals, a time-related increase of both mercury and selenium was found in liver and kidney after the administration of methylmercury, while in the other tissues examined, e.g. brain, thyroid, blood, only the concentrations of mercury increased. In the wild seals, it was also demonstrated that the selenium concentrations showed a positive correlation with the concentrations of mercury. Atomic ratios of mercury and selenium were close to one in the wild seals as was also found in previous studies. However, atomic ratios above one were found in seals fed additional methylmercury. In vitro studies with liver homogenates of seals did not provide evidence for the presence of a biochemical demethylation mechanism nor for any effect of selenium on the demethylation process.