In view of the relative paucity of information on trace metal distribution in marine animals, a series of studies were Initiated in order to investigate the mechanics of concentration of these materials. Data were obtained on a number of mollusk species. There is a wide variation in species ability to take up and concentrate zinc, lead, nickel, cobalt, iron, manganese, copper, cadmium, and chromium within their natural (estuarine) environment. Uptake rates in a controlled, simulated-environment system using various concentrations of lead, copper, cadmium, and zinc indicate that all of the species studied varied in their selectivity for the particular metal taken up. The rate of uptake, and the tissue level attained, were found to vary with time, and the particular metal concentration used. Using various lead concentrations it was observed that of the various anatomical areas, the muscle, mantle edge, mantle, remainder, gill, gonad, and digestive gland accumulated increasing tissue levels in the order given. Depletion is a relatively slow process in most cases; it varies from species to species as to rate and final tissue concentration.