In order to study elution and displacement of adsorbed ions into filtered sea water and their displacement by adsorbed Cd, analyses for Cd, Ag, Mn, Pb, V, and Zn were carried out on freshly killed oysters, specimens maintained for 100 hr in filtered sea water and on a further number treated for the same period with filtered sea water containing 50 p.p.m. Cd. Although a high Cd concentration did not appear to displace other ions, there was some evidence for loss of ions from the visceral mass. Concentrations in oysters equilibrated with sea water containing added Cd, indicated that this element was strongly accumulated in the gills, visceral mass, and heart. These findings were confirmed in radiometric experiments involving uptake of 113/113mCd from sea water containing this radionuclide. These experiments were also used to measure the rate of uptake of Cd which although relatively slow, was appreciable in magnitude. Fractionation factors for the various organs for varying Cd concentrations in sea water showed a very similar pattern for all organs studied and indicated cessation of uptake at Cd concentrations between 40 and 140 p.p.m. It was concluded that uptake was due to unselective adsorption probably by coordination to organic ligands. This theory was strengthened by radiometric experiments on the oyster heart where proteinbound Cd probably did not exceed 25 % of the total.