The switching of ADP-ribosylation factors from the inactive form to the active form is catalyzed by ARF-GEF (ADP ribosylation factor--guanine nucleotide exchange protein) proteins containing a Sec7 domain. The murine Arfgef2 gene encoding the BIG2 protein belongs to the class of high molecular mass (>100 kDa) ARF-GEF proteins. BIG2 is believed to be associated with the trans-Golgi network and the recycling endosomes. In humans, mutations in the ARFGEF2 gene cause autosomal recessive periventricular heterotopia with microcephaly. To elucidate the function of BIG2 in mouse we studied a gene-trap mouse line with a functional disruption of the Arfgef2 gene. Heterozygous mutants did not reveal phenotypic abnormalities and were fertile. However, no homozygous embryos were obtained from breeding heterozygous females and males. To explore the reason for embryonic lethality, we analysed the pattern of expression of Arfgef2. Arfgef2 transcripts were detected in several adult tissues. Interestingly, Arfgef2 undergoes alternative splicing and the splicing pattern differs among tissues from adult animals. Moreover, the LacZ reporter gene of the gene-trap construct was used to reveal the expression of Arfgef2 during embryonic development. Here, we show that Arfgef2 mRNA is stored in the oocyte and is likely translated during the first embryonic divisions. SNP (Single Nucleotide Polymorphism) markers were used to demonstrate that the embryonic Arfgef2 gene is activated first at the 4-cell stage, suggesting an important role for embryonic development. This assumption is supported by the failure of Arfgef2-deficient oocytes fertilized with Arfgef2-deficient sperm to develop into 4-cell stage embryos. Our results indicate that murine BIG2 is essential for early embryonic development.