Experiments were performed to determine whether mercury and lead in the diet of captive mallards (Anas platyrhynchos) would increase the eggshell thinning above that caused by DDE alone. DDE alone caused a 15% reduction in shell thickness, but neither Pb and Hg contributed significantly to mallard eggshell thinning, nor did these compounds appear to act synergistically with DDE. The fact that Hg and Pb did not cause eggshell thinning in captive mallards leads researchers to the conclusion that wild avian species are probably not affected by Hg and Pb. The residues found in the various tissues analyzed are summarized.