Patulin (PAT), a highly toxic, carcinogenic, heterocyclic lactone is produced by a variety of fungal species, including Penicillium and Aspergillus. This compound has been isolated from various apple products and is stable in apple and grape juice and dry corn. It has been reported to be cytotoxic and to exert adverse influence on development in vivo in mice and merits further study and evaluation. In this study, whole rate embryo culture (WEC) was used to determine the teratogenic potential of PAT in vitro. Embryos were exposed to PAT-treated (0.00-62 microM) rat serum for 45 h. The embryos that were exposed to 62 microM PAT were not evaluated because they did not survive beyond 40 h of incubation. The results indicate that PAT induced a statistically significant reduction in protein and DNA content, yolk sac diameter, crown rump length, and somite number count. Patulin treatment also resulted in an increase in the frequency of defective embryos. Anomalies included growth retardation, hypoplasia of the mesencephalon and telencephalon, and hyperplasia and/or blisters of the mandibular process. Thus, the data from the present study provide further evidence supporting the conclusion that the whole rat embryo assay is a rapid and sensitive in vitro method that can be employed to pre-screen developmentally toxic mycotoxins.