Oysters demonstrated an ability to significantly concentrate dieldrin and endrin. Concentration ratios obtained after 168-hr exposures to endrin were 1670 at 0.1 mug/L and 2780 at 50 mug/L. Dieldrin was concentrated to higher levels. Exposure to 14C-labelled dieldrin at 0.5 mug/L produced whole body concentrations 2880 times the ambient level at 168 hr, while exposure to nine mug/L of dieldrin resulted in a concentration ratio of 2070 following the same period of exposure. Both endrin and dieldrin showed distinct linear regions in semi-logarithmic plots of uptake against time. Initial uptake was rapid and was followed by somewhat slower but still rapid uptake over the next 6 to 48 hr. Uptake within each of the stages followed an exponential form.