Journal of Environmental Radioactivity 46 (1999) 171}177 Caesium retention during pregnancy in mice I.E. Stamatelatos*, J. Kalef-Ezra, S. Economides, S. Yasumura  Institute of Nuclear Technology and Radiation Protection, National Center for Scientixc Research &&Demokritos'', P.O. Box 60228, Aghia Paraskevi, 153 10, Greece  Medical Physics Laboratory, Medical School, University of Ioannina, Ioannina, 451 10, Greece  Medical Department, Brookhaven National Laboratory, Upton, New York, 11973, USA Received 29 May 1998; accepted 10 November 1998 Abstract The e!ect of pregnancy on caesium retention in mice was assessed. Fifteen female mice were injected with caesium-137 solution intraperitoneally and measurements of body radioactivity were performed. For the control group of mice (n"8), caesium retention could be adequately described, for the time period from day ¹"7 to 26 post-injection, by a two exponential model, where 90% of caesium was eliminated by a component with a biological half-time of (2.83$0.78) d and 10% by a component with a biological half-time of (16.3$7.3) d. However, an alteration of caesium elimination rate, which a!ects the component with the longer half-time was observed in the group of pregnant mice (n"7). The observed e!ect was a decrease in excretion rate, in contrast to recent human observations where an increase was found. This di!erence in mice is associated with changes in metabolic rate, and, to a "rst approximation, is related to the gain in mass during pregnancy.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Caesium; Mice; Pregnancy; Radioactivity 1. Introduction Measurements of whole-body radioactivity were made at the University of Ioan- nina's whole body counter facility in order to assess the radiation body burden to the Greek population resulting from the nuclear reactor accident at Chernobyl (Kalef- Ezra, Hatzikonstantinou, Leontiou & Glaros, 1992). Among the "ndings it was shown that there is a signi"cant decrease in total body caesium during pregnancy (Kalef-Ezra & Yasumura, 1997). This was in agreement with the observations of other workers who have also reported a decrease in the biological half-time of caesium during * Corresponding author. Tel.: 00 301 650 3718; fax: 00 301 653 3431; e-mail: ion@ipta.demokritos.gr. 0265-931X/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0265-931X(98)00123-4