Pattern formation in the mouse preimplantation embryo is tightly regulated and essential for successful development. Wnt genes are known to regulate cell interactions and cell fate in invertebrates and vertebrates and, therefore, may play a role in the specification of cell lineages and cellular interactions that occur in preimplantation development. Using degenerate primers based on conserved protein sequences in Wnt coding regions, we have found evidence for Wnt gene expression at the blastocyst stage of mouse preimplantation development. We have identified sequences encoding Wnts3a and 4 and confirmed that these are present as transcripts in early development by using reverse transcriptase-polymerase chain reaction (RT-PCR) with specific primers located in the 5' half of these Wnt genes. Studies on the timing of expression showed that Wnt3a transcripts were present in 2-cell embryos which may represent maternally or embryonically derived transcripts since the major transition of maternal to zygotic gene expression occurs during the late 2-cell stage. Both Wnt3a and 4 transcripts were detected in some precompact 4/8-cell stages with consistent expression detected in all compact 8-, 16-cell and blastocyst stages. To our knowledge, expression of Wnt genes has not been previously described at such an early stage of mammalian development.