To determine how quickly breeding birds would have to feed in a mercury-contaminated area before harmful concentrations of mercury, as methylmercury, built up in their eggs, we fed female mallards(Anas platyrhynchos) a control diet or diets containing 0.5, 1, 2, 4, or 8 mg/g mercury (on what was close to a dry weight basis) as methylmercury chloride for 23 d.