In situ hybridization has been used to localize RNAs for a variety of non-collagenous proteins during embryogenesis of the mouse, with particular reference to bone formation. Transcripts of the Sparc (osteonectin) gene are first detected around 14.5d post coitum (p.c.) in the periosteal layer around the cartilage model of long bones and in membrane bone in the head. By 15.5d p.c. transcripts are seen in hypertrophic cartilage and later in cells within the bone closely associated with new matrix. By contrast, RNA for 2ar (osteopontin) is not seen in the periosteal layer but is confined to cells in the marrow region. TIMP (tissue inhibitor of metalloproteinase) transcripts show a very similar distribution to those for Sparc.