Alpha-fetoprotein (AFP) is a synthetic product of only the visceral yolk sac endoderm and fetal liver during mouse embryogenesis. To examine the involvement of transcriptional and post-transcriptional mechanisms in the regulation of tissue-specific expression of the AFP gene, the distribution of AFP mRNA in tissues at different stages of development was determined. Total RNA was isolated from mid-gestation tissues, and a microextraction procedure was developed for earlier stage embryos where the amount of tissue was limited. AFP mRNA was measured by a dot-hybridization assay using a 32P-labelled AFP cDNA clone as probe. Results show that only visceral yolk sac endoderm and fetal liver of midgestation embryos contained AFP mRNA. At earlier stages AFP mRNA was confined to regions within the visceral endoderm which have previously been shown to be AFP positive. These results indicate that expression of the AFP gene during post-implantation development is probably controlled at the level of AFP gene transcription. In addition, we show by in situ hybridization to tissue sections that all endoderm cells of the visceral yolk sac contain AFP mRNA, indicating that the visceral endoderm layer is a homogeneous population of cells with respect to transcription of the AFP gene.