The expression of the mRNA coding for glyceraldehyde-3-phosphate dehydrogenase has been studied during embryonic development of the chicken. In each tissue examined, only one mRNA species coding for a single glyceraldehyde-3-phosphate dehydrogenase subunit could be detected using translation in vitro with RNA blotting and hybridization. The mRNA species coding for glyceraldehyde-3-phosphate dehydrogenase subunits in different chicken tissues have identical electrophoretic mobilities suggesting that they are structurally very similar if not identical.