Studies with different avian species have revealed that surface applications of microliter amounts of some crude and fuel oils that coat less than 70% of the egg surface result in considerable reduction in hatching with teratogenicity and stunted growth. Other stUdies have shown that the embryo toxicity is dependent on the aromatic hydrocarbon content, further suggesting that the toxicity is due to causes other than asphyxia. In the present study the effects of three polycyclic aromatic hydrocarbons identified in petroleum were examined on mallard (Anas platyrhynchos) embryo development. Addition of benzo[a]pyrene (BaP), chrysene, or 7,7 2-dimethylbenz[ a]anthracene (DMBA) to a synthetic petroleum hydrocarbon mixture of known composition and relatively low embryotoxicity resulted in embryo toxicity that was enhanced or equal to that of crude oil when 10 :I was applied externally to eggs at 72 h of development. The order of ability to enhance embryo toxicity was DMBA > BaP > chrysene. The temporal pattern of embryonic death was similar to that reported after exposure to crude oil, with additional mortality occurring after outgrowth of the chorioallantois. Retarded growth, as reflected by embryonic body weight, crown-rump length, and bill length, was accompanied by teratogenicity. Abnormal embryos exhibited extreme stunting; eye, brain, and bill defects; and incomplete ossification. Gas chromatographic-mass spectral analysis of externally treated eggs showed the passage of aromatic hydrocarbons including chrysene through the shell and shell membranes to the developing embryos. These findings suggest that the presence of polycyclic aromatic hydrocarbons in petroleum, including BaP, chrysene, and DMBA, significantly enhances the overall embryotoxicity in avian species.