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Pregnant ewes have been chronically contaminated during pregnancy and lactation with Cs-134 administered as CsCl in the food. Plateau concentrations in excreta and blood were reached after about 20 d of contamination. At delivery, the contamination levels in lamb's tissue were lower than those in the corresponding organs from an ewe; they increased rapidly as the lambs received milk from their contaminated dam, up to higher levels than in ewes. The transfer coefficient were estimated in various tissues from an ewe slaughtered at delivery (muscles 0.57 d/kg, kidneys 0.50 d/kg, liver 0.33 d/kg, blood 0.02 d/kg) and milk (0.09 d/l). During the decontamination phase, the radioactivity was excreted from the ewe's body according to a two exponential pattern with half-times of about 1.3 and 20 d. The retention in muscles, kidneys and liver from the ewes was characterised by two compartments with longer half-times of 2.3 to 2.9 d and 123 to 174 d respectively. The contamination levels in organs from lambs separated from their dam 3 days after birth and fed uncontaminated artificial milk decreased with a global half-time of about 10 d.