[en] Strontium-90 is a hazardous fission product from nuclear reactions. Its transplacental passage is of particular interest because of the high sensitivity of embryos/foetuses to ionizing radiation. We have used whole-body autoradiography to study the foetal uptake and retention of ⁹⁰Sr and the embryonic/foetal transfer of this isotope as a function of gestational age. Fifteen minutes after an intraperitoneal injection of 50 μci⁹⁰Sr-nitrate into a 14-day-pregnant mouse, radioactivity was present in the placenta, but not in the foetus. During a 72-hour observation period, ⁹⁰Sr gradually accumulated in the hard foetal tissues. In the soft foetal tissues, accumulation occurred up to 12-24 hours. After 72 hours, no ⁹⁰Sr could be detected in the soft tissues. ⁹⁰Sr crossed the visceral yolk sac placenta during early gestation, the visceral yolk sac and the chorioallantoic placentas during late gestation. Transfer increased considerably at the end of the latter period. There was a marked uptake of ⁹⁰Sr in all structures of early embryos, whereas the isotope concentrated in the preskeletal and skeletal parts of the foetuses. Intake of ⁹⁰Sr during early gestation may therefore be more injurious to the offspring than intake during late gestation. (author)