In mouse development, parietal endoderm (PE) is formed from both primitive endoderm (PrE) and visceral endoderm (VE). This process can be mimicked in vitro by using F9 embryonal carcinoma cells (EC) cells, differentiated to PrE or VE cells, and treating these with Parathyroid Hormone related Peptide (PTHrP). By means of differential display RT-PCR, we identified Snail (Sna) as a gene upregulated during the differentiation from F9 PrE to PE. We show that Sna is an immediate early target gene of PTHrP action in the formation of F9 PE cells. Using RT-PCR, we detected Sna transcripts in pre-implantation mouse embryos from the zygote-stage onwards. Sna was strongly upregulated in parallel with type 1 PTH/PTHrP Receptor (PTH(rP)-R1) mRNA in mouse blastocysts plated in culture, concomitant with detection of the PE-marker Follistatin and appearance of PE cells. By radioactive in situ hybridization on sections of mouse embryos, we found Sna expression in the earliest PE cells at E5.5. Sna remained expressed until at least E7.5. At this stage, we also observed clear expression in endoderm cells delaminating from the epithelial sheet of VE cells in the marginal zone. We conclude that PTH(rP)-R1 and Sna are expressed in endodermal cells that change from an epithelial to a mesenchymal phenotype. Since Sna expression has been described at other sites where epithelio-mesenchymal transitions (EMT) occur, such as the primitive streak at gastrulation and in pre-migratory neural crest cells, we hypothesize that Sna is instrumental in the action of PTHrP inducing PE formation, which we propose to be the first EMT in mouse development.