High concentrations of boron (B) and selenium (Se) have been found in aquatic food chains associated with irrigation drainwater. Total biomass of invertebrates, a maJor source of protein for wild ducklings, is sometimes diminished in agricultural drainwater ponds contaminated with Se and B. Dayold mallard (Anas platyrhynchos) ducklings received an untreated diet (controls) containing 22% protein or diets containing 15 ppm (microgram/g) Se (as selenomethionine), 60 ppm Se, 1,000 ppm B (as boric acid), 15 ppm Se with 1,000 ppm B, or 60 ppm Se with 1,000 ppm B. In a concurrent experiment, the above sequence was repeated with a proteinrestricted (7%) but isocaloric diet. After four weeks, blood and tissue samples were collected for biochemical and histological examination. With 22% protein and 60 ppm Se in the diet, duckling survival and growth was reduced and histopathological lesions of the liver occurred. Boron alone caused some reduction in growth. Several interactive effects occurred between B and Se, including further reduction in growth, and increases in plasma glutathione reductase activity, hematocrit, hemoglobin and plasma protein concentrations. With 7% protein, the growth of controls was less than that with 22% protein, 60 ppm Se caused 100% mortality, and growth effects of 15 ppm Se and 1,000 ppm B alone were more pronounced than with 22% protein. Selenium accumulation increased in the liver with 7% protein. Interactive effects were greater for Se and B with 7% protein than with 22% protein and included significant mortality and enhanced accumulation of Se in the liver. These findings suggest the potential for more severe toxicological effects of Se and B independently and interactively on duckling survival and development when dietary protein is diminished.