AbstractWe measured hepatic enzyme activity in 25‐d embryos from eight colonies in the Great Lakes and one relatively uncontaminated colony in the Bay of Fundy. This was done to further investigate the use of the mixed‐function oxidase system of herring gull (Larus argentatus) embryos for bioeffects monitoring. The changes in ethoxyresorufin o‐dealkylation (ERoD) and the rate of aminopyrene‐N‐demethylation (APDM) across the Great Lakes were small. However, the rate of aniline hydroxylation (AnH) was significantly depressed in seven of eight colonies when compared with control colony. This suppression was greatest in embryos from the three colonies in Lake Ontario, where mirex levels were highest. Organochlorines were measured in egg homogenates from each colony. There was a significant negative correlation of the rate of APDM with both polychlorinated biphenyl and hexachlorobenzene levels and also of the rate of AnH with mirex levels. In conjunction with the above, an egg injection experiment using three doses each of hexachlorobenzene and mirex was conducted to elucidate these compounds' effects on the enzymes measured. The effects on body weight, liver weight and enzyme activity were small compared with the mortality incurred in the eggs injected with hexachlorobenzene. The embryonic LD50 for hexachlorobenzene was calculated as 4.3 ppm. Mortality in the controls injected with 1,4‐dioxane was 20%, and that of eggs injected with mirex was similar to that of controls.