Technical Abstract: Planar PCB congeners are embryotoxic and teratogenic to birds including Americal Kestrels. The development toxicity of 3,3', 4'4, 5-pentachlorobiphenyl (PCB 126) was studied for the first 10 days post hatching in nestling Kestrels. Dosing with 50 ng/g of PCB 126 resulted in a liver concentration of 156 ng/g, with liver enlargement, mild coagulative necrosis, and over ten-fold increases in hepatic microsomal ethoxyresorufin-o-deethylase (EROD) and benzyloxyresorufin-o-dealkylase(BROD), and approximately 5-fold increase in methoxyresorufin-o-dealkylase (MROD). At higher concentrations, greater effects were obvserved, including decreased bone growth, and decreased spleen. Highly significant correlations were seen between increasing liver concentrations of PCB 126 and ratio of oxidized to reduced glutathione. These findings indicate that nestling kestrels are more susceptible to PCB 126 toxicity than adults but less than embryos, and that planar PCB's are of potential hazard to nestling birds including eagles.