The placental transfer of F18as fluoride ions has been visualized autoradiographically in mice up to one hour following intravenous injection of the isotope. The foetal skeleton is seen to accumulate much less F18than the maternal skeleton.Following the simultaneous ingestion of F18+Ca45an autoradiographic technique, which gives roughly similar uptake-pictures for both isotopes in the maternal skeleton, has produced uncomparably stronger Ca45-pictures of the foetal skeletons.The often occurring soptty concentrations of F18in the fullterm mouse placenta were found to coincide with the concentrations of simultaneously injected Ca45, and to relate to the general areas of the degenerative placental calcifications.A quantitative study of the placental transfer in mice of simultaneously administered F18and Ca45showed a much smaller percentage of the F18and Ca45-dose in the foetuses up to 4 hours, in spite of a more complete absorption of ingested F18.The resulted are discussedi.al.from a clinical point of view and are viewed in relation to clinical data on record in the literature.