Classical evaluations of the effects of toxicants on development in birds for ecological impact assessments and teratogenic evaluation generally entail an evaluation of the late embryo, immediately before hatching, or young hatchling. In the laboratory, these studies generally require approximately three weeks before one can evaluate the late embryo or hatchling, and a full teratogenic evaluation of growth and developmental parameters is relatively time intensive. Recent studies have shown that many teratogenic abnormalities may be detected as soon as the affected organ system starts to develop or differentiate. The early embryo teratogenesis assay is being developed to reduce the amount of time needed to evaluate potential teratogenic or developmental toxicological effects. Embryos which have been exposed in the laboratory or in the natural environment are incubated (preferably in the laboratory) and sacrificed at known time points which correspond to the time when specific developmental indicators (such as the heart or visceral arches) have begun to develop. These embryos are then evaluated for the type and frequency of abnormalities manifested at each stage of development. We have begun a series of laboratory studies using both a laboratory model animal (the domestic chicken) and wildlife species (barn and tree swallows) which enable us to compare the early embryo abnormalities induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; chicken) or a mixture of environmental contaminants, which include TCDD-like compounds (swallow), to the abnormalities observed in the late embryo, hatchling or nestling.