Bipyridylium herbicides produce embryotoxic and teratogenic effects in dipteran, amphibian, avian, and mammalian organisms. Diquat dibromide, a bipyridylium compound, is commonly used as an aquatic herbicide. Mallard (Anas platyrhynchos) eggs were exposed to diquat by immersing the eggs for 10s in solutions of 0.88, 3.5, 7, 14, or 56 g/L on either the fourth or twenty-first day of incubation. Application of diquat on day 4 yielded an estimated LC50 of 19.5 g/L through 18 days of incubation, and 9.6 g/L through hatching. Body and organ weights, and bone lengths of hatchlings did not differ between control and treatment groups with the exception of a slight increase in brain weight in the 14 g/L group. Malformations in diquat-treated embryos included defects of the brain, eye, bill, limb, and pelvis; skeletal scoliosis; and incomplete ossification. Subcutaneous edema was also present. Significant manifestations of oxidative stress were apparent in hatchlings and included increased hepatic thiobarbituric acid reactive substances (TBARS) (lipid peroxidation) and decreased brain reduced glutathione (GSH). Brain protein-bound sulfhydryls (PBSH) increased. Diquat applied on day 21 of incubation yielded an estimated LC50 of 12.6 g/L through hatching. Exposure at this late stage of development did not produce deformities. Body and organ weights, and, bone lengths of hatchlings did not differ between control and treatment groups. Significant manifestations of oxidative stress in hatchlings included decreased brain GSH, increased oxidized glutathione (GSSG) and ratio of GSSG:GSH. This study suggests that concentrations of diquat commonly used for aquatic weed control, when based upon the expected dilution effect of average water depth of the application area, would probably have little impact on mallard embryos. However, concentrations applied above ground to weeds and cattails along the edge of waters and ditches could adversely affect the survival and development of mallard embryos, and presumably other avian species nesting in such habitats.