Black-crowned night-heron (Nvcticorax nvcticorax) pipping embryos were studied from undisturbed (Chincoteague National Wildl ife Refuge, VA) and industrialized (Cat Island, Green Bay WI, and Bair and W. Marin Islands, San Francisco Bay, CA) locations. Hepatic aryl hydrocarbon hydroxylase (AHH) , ethoxyresorufin-O-dealkylase, (EROD), benzyloxyROD (BROD), pentoxyROD (PROD) and ethoxycoumarinOD (ECOD) activities and burdens of organochlorines (embryo + yolk sac - liver) were quantified. AHH, BROD, ECOD and EROD were induced up to 100-fold (P<.O5) in embryos from Cat Island compared to the other sites. Greatest burdens of total PCBs and p,p?DDE were detected in Cat Island embryos. Monooxygenase activities (AHH, BROD, ECOD and EROD) and PCB concentrations were significantly correlated (r=O.50 to 0.72). These and other data indicate that monooxygenases may be rapid and inexpensive biomarkers of exposure to some PCB congeners. Current efforts include determination of PCB congeners and other contaminants in these embryos, additional characterization of the induced P-450 isozymes, and expanding the study to include heron embryos and nestlings at other estuaries.