Pea Patch Island in Delaware Bay is the site of the largest heronry north of Florida. From 1989-93. the population of 9 species of wading birds numbered approximately 12,000 pairs. but has recently declined to 7,000 pairs. Because Delaware Bay is a major shipping channel. and receives anthropogenic releases of toxic substances from agricultural, industrial and municipal point and nonpoint sources, contaminant exposure and effects to the heronry have been an ongoing concern. In 1997, piping black-crowned night-herons (BCNHs) wee collected from separate nests at Pea Patch Island (N=l5), and from a coastal reference site, Middle Island (N=9), in Rehoboth Bay. DE. There was neither evidence of malformations nor hepatic histopathological lesions, and body and liver weights did not differ between sites. Biomarkers of petroleum hydrocarbons, polyhalogenated contaminant and metal exposure (cytochrome P450 induction and oxidative stress responses) did not differ (P>0.05) between sites, however, activities of benzyloxy- and ethoxyresorufin-O-dealkylase were greater in 3 of 15 embryos from Pea Patch Island compared to Middle Island. Concentrations of 21 organochlorine pesticides and metabolites were relatively low at both sites. with p,.p'DDE values well below the threshold associated with eggshell thinning. Although total PCB concentration was modestly elevated (P<0.05) in Pea Patch Island BCNH embryos, levels of axylhydrocarbon-receptor active PCB congeners. dioxins, dibenzofurans and Toxic Equivalents were low and did not differ between sites. Surprisingly, organochlorine contaminant exposure and effects in Delaware Bay BCNHs appear to be considerably less than that observed and recently reported (ETC 16:2315-2322,1997) for herons residing in the Chesapeake Bay.