This study aims to elucidate gene expression of laminin and its role in expansion of the blastocyst during mouse early embryo-genesis. The gene expression of laminin, in particular the B1 subunit and the synthesis of laminin polypeptides, was examined during the expansion of blastocyst by a RNA-blot hybridization with 32P-labeled laminin B1 cDNA and immunoprecipitation followed by a SDS-PAGE, respectively. Laminin B1 transcript was actively expressed in the blastocyst stage of embryos. The gene expression of laminin B1 and the synthesis of laminin protein were also increased when blastocyst was expanded. Treatments of cAMP analogue, isobutylmethylxanthine, forskolin, and cholera toxin, which are known to stimulate the blastocyst expansion, increased laminin B1 transcript levels and synthesis of laminin polypeptides. Treatment with retinoic acid, a known regulator of laminin gene expression, not only increased the gene expression of laminin but stimulated the blastocoel expansion without a significant increase in intracellular cAMP levels. These results indicate that laminin gene expression may play an important role in the process of blastocyst expansion in the mouse preimplantation embryos.