A study of wood ducks (Aix sponsa) was conducted along the Coeur d'Alene River system in northern Idaho in 1986 and 1987. Most of this area has been subjected to severe contamination from lead and other metals from mining and smelting since the 1880s. In 1986, a preliminary study of wood duck nesting was conducted in the contaminated area; incubating hens captured in nest boxes were bled and weighed. Blood samples were used to determine lead and cadmium concentrations and physiological characteristics. In 1987, an intensive study of wood ducks involved trapping and monitoring nest boxes in the contaminated area. Blood and tissue samples were also taken from wood ducks from a reference area without known contamination from metals. Lead levels in blood and tissues of most wood ducks from the contaminated area frequently exceeded those considered hazardous to birds; maximum levels (wet weight) of lead were 8 :g g?1 in blood and 14 :g g?1 in liver. Changes in physiological characteristics constituted the only evidence of potentially adverse effects from lead. In the contaminated area, nesting success (55% unadjusted, 35% Mayfield estimate) was less than in other areas where predation was low and nest boxes were used; but lead concentrations and physiological characteristics of blood were similar in successful and unsuccessful hens. Values of ALAD, hemoglobin, and body mass were negatively correlated with blood concentrations of lead, whereas protoporphyrin was positively correlated with lead levels in the blood. Some of the protoporphyrin values (1,091 :g dl?1 in a male and 756 :g dl?1 in a female) equalled those associated with lead toxicosis in experimental birds. ALAD activity was low in most birds from the contaminated area; values of 0 were obtained from 11 birds. Lead levels in blood, ALAD, protoporphyrin, and hemoglobin were significantly different between birds from the contaminated and reference areas. Concentrations of lead in ingesta of wood ducks ranged from 0.9 to 610 :g g?1 in the contaminated area and 0.2 to 0.6 :g g?1 in the reference area. Levels of cadmium in kidneys of wood ducks ranged from 1g g?1 to 20 :g g?1 in the contaminated area and from only to 0.1 :g g?1 to 1 :g g?1 in the reference area. Cadmium concentrations were less than known effect levels.