[en] In the present work different concentrations of methyl-³ H-thymidine was added to the culture medium micro drops containing the mouse zygotes at pro nuclear stage and the embryos were cultured in vitro at 37⁰ C in a humidified atmosphere with 5% of CO₂ for four days up to the expanded blastocyst stage, the established end point to calculate the L D₅₀ lethal dose. The blastocyst formation rate decreased with increasing concentration of tritium in medium and a value of 2.4 X 10³ Bq/ml for L D₅₀ was obtained. The ³ H-Td R incorporation by the embryo during the preimplantation period was low at the beginning and increased quickly during the morula and the blastocyst development. (author)