Frog oocytes and embryos have long served as traditional subjects of embryological research providing structural and functional information for the interpretation of the biological processes underlying development. A large number of various chemical agents induce typical teratological changes in frog embryos. However, the effects of metal deficiency of the first transition and IIB series or of chelating agents specific for these metals have never been examined in the frog. Multidentate chelating agents, including 1,10-phenanthroline (OP), which coordinate metals through N, O or S donor atoms are teratogenic also but in a manner characteristic for this class of reagents and completely different from those referred to above. Exposure to 10(-5) M OP causes maximal malformations with minimal mortality inducing craniofacial and skeletal abnormalities with failure of eye, head and other organ formation in 74% of frog embryos. In contrast, the non chelating analogue 1,7-phenanthroline (MP) has no effect at this concentration. A concentration of 10(-3) M OP is lethal. The known characteristics of either zinc and/or iron complexes with OP as well as the concentrations of these elements in frog oocytes and embryos are consistent with the hypothesis that the teratological observations are due to an effect of OP on either zinc or iron proteins.