The National Contaminant Biomonitoring Program of the U.S. Fish and Wildlife Service Includes the sampling of freshwater fish, starlings, and duck wings. In order to include an estuarine component, herons and egrets are being evaluated. Organochlorine concentrations were measured In eggs and chicks of Snowy Egrets Egretta thula, Great Egrets Casmerodius albus, and Black-crowned Night-Herons Nycticorax nycticorax collected In Rhode Island, Texas, and California. Chicks of all species at all locations accumulated DDE and polychlorinated biphenyls (PCBs), and accumulation rates may serve as the basis for an indicator program of estuarine contamination. Biochemical re-sponses of embryos and chicks were also evaluated. Microsomal arylhydrocarbon hydroxylase and benzyloxyresorufin-O-dealkylase activities in livers of 10-day-old night-heron chicks were elevated at a contaminated site in San Francisco Bay, CA, when compared to a 'reference' site in Vlrginia. Quantification of liver enzyme activities may serve as a rapid cost-effective biomarker of pollutant exposure.