We have examined by Northern analysis and in situ hybridisation the expression of TGF β1 β2and β3 during mouse embryogenesis. TGF β1 is expressed predominantly in the mesodermal components of the embryo e.g. the hematopoietic cells of both fetal liver and the hemopoietic islands of the yolk sac, the mesenchymal tissues of several internal organs and in ossifying bone tissues. The strongest TGF β2 signals were found in early facial mesenchyme and in some endodermal and ectodermal epithelial cell layers e.g., lung and cochlea epithelia. TGF β3 was strongest in prevertebral tissue, in some mesothelia and in lung epithelia. All three isoforms were expressed in bone tissues but showed distinct patterns of expression both spatially and temporally. In the root sheath of the whisker follicle, TGF β1 β2 and β3 were expressed simultaneously. We discuss the implication of these results in regard to known regulatory elements of the TGF β genes and their receptors.