We have compared the expression of the genes encoding transforming growth factors beta 1, beta 2 and beta 3 during mouse embryogenesis from 9.5 to 16.5 days p.c. using in situ hybridisation to cellular RNAs. Each gene has a different expression pattern, which gives some indication of possible biological function in vivo. All three genes appear to be involved in chondroossification, though each is expressed in a different cell type. Transcripts of each gene are also present in embryonic epithelia. Epithelial expression of TGF beta 1, beta 2 and beta 3 RNA is associated with regions of active morphogenesis involving epithelial-mesenchymal interactions. In addition, widespread epithelial expression of TGF beta 2 RNA can be correlated with epithelial differentiation per se. The localisation of TGF beta 2 RNA in neuronal tissue might also be correlated with differentiation. Finally both TGF beta 1 and beta 2 transcripts are seen in regions actively undergoing cardiac septation and valve formation, suggesting some interaction of these growth factors in this developmental process.