The effects of the four metals, copper, zinc, cadmium and manganese on the common cockle, Cerastoderma edule L., were investigated. With regard to both the inhibition of filtering rate and mortality of cockles, copper was the most toxic metal, followed by zinc then cadmium. Manganese was the least toxic and appeared to have no adverse effect on the animals. The accumulation of the metals by the cockles from a range of added concentrations was studied by analysing their soft tissues using atomic absorption spectrophotometry. Copper, zinc and cadmium were all accumulated markedly, but the relationships between accumulation, added metal concentration and duration of exposure were complex. Manganese accumulation was almost undetectable. A major aim of the study was to investigate the effect of one metal on the accumulation of another. The inherent variability in these stable metal experiments made this very difficult. A technique was devised where the uptake of radioisotopes of the metals by excised and isolated cockle gills was measured. This reduced variability and was used in all further investigations. Unlike the uptake of zinc, cadmium and manganese, where uptake was proportionately less at higher added concentrations, the uptake of copper by the gills was proportionately greater at higher added levels. Uptake of manganese was much lower than that of the other metals. Experiments were performed where the amount of metal that was surface-bound to the gills and that which was internally bound was determined. With all the metals tested, the proportion of metal taken up that was bound internally increased with time. These findings are discussed in terms of the involvement of metal-binding ligands and the responses of whole animals to the metals. The excised gill method was used to examine interactions between the metals and it was found that zinc and copper generally reduced the uptake of cadmium and manganese. The latter two metals also inhibited each other's uptake. Severity of metal to metal interactions increased with time, emphasising their intracellular nature. The results are discussed with reference to the importance of metal interactions in polluted field situations and the desirability of setting flexible emission standards. These results, along with those from experiments on metal loss from gills and those from further experiments on interactions were used to produce a simplified model of metal uptake. It is proposed that at least two types of intracellular metal-binding ligands are involved, which have different affinities for the metals. Such ligands may be part of a metal detoxification mechanism on which metal resistance depends. The final part of the study involved comparing the responses to metals of cockles from two localities. It was shown that animals from the more polluted site displayed some tolerance of copper and zinc.