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In 1993, great blue heron (Ardea herodias; GBH) eggs were collected from 10 colonies on the upper Mississippi River (UMR). They were then artificially incubated until pipping and analyzed for mercury, selenium, and organochlorines. Livers of embryos were analyzed for hepatic microsomal ethoxyresorufin‐O‐dealkylase (EROD) activity and four measures of oxidative stress. Brains were measured for asymmetry and blood was measured for the coefficient of variation of DNA (DNA CV). Organochlorine concentrations were generally low (geometric mean DDE = 1.3 μg/g wet weight; polychlorinated biphenyl [PCB] = 3.0 μg/g; 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin [TCDD] = 11.5 pg/g). Eggshell thickness was negatively correlated with DDE concentrations. Mercury (geometric mean = 0.8 μg/g dry weight) and selenium (3.1 μg/g dry weight) concentrations in GBH eggs were within background levels. EROD activity was …