Eighty pairs of mallards (Anas platyrhynchos) were fed an uncontaminated diet until each female had laid 15 eggs. After each female had laid her 15th egg, the pair was randomly assigned to a control diet or diets containing 5, 10, or 20 ?g/g mercury as methylmercury until she had laid a second set of 15 eggs. There were 20 pairs in each group. After the second set of 15 eggs, the pair was returned to an uncontaminated diet, and the female was permitted to lay another 30 eggs. For those pairs fed the mercury diets, the even-numbered eggs were incubated and the odd-numbered eggs were saved for possible mercury analysis. Mercury in the even-numbered eggs was estimated as the average of what was in the neighboring odd-numbered eggs. Neurological signs of methylmercury poisoning were observed in ducklings that hatched from eggs containing as little as 2.3 ?g/g estimated mercury on a wet-weight basis, and deformities were seen in embryos from eggs containing about 1 ?g/g estimated mercury. Although embryo mortality was seen in eggs estimated to contain as little as 0.74 ?g/g mercury, there were considerable differences in the sensitivity of mallard embryos, especially from different parents, with some embryos surviving as much as 30 or more ?g/g mercury in the egg.