The postnatal development and differentiation of the tracheal epithelium in Syrian golden hamsters was examined by SEM. Surface structures indicate a proliferative growth for the first 10 days after delivery, but differentiation into a mature respiratory epithelium is still incomplete on the 30th day post partum. Cells with microvilli (less differentiated cells, mucus cell precursors and mucus-producing cells) outnumbered ciliated cells up to the 20th day. The topographical distribution of ciliated cells was different at various stages of development in the pars membranacea, pars intercartilaginea and pars cartilaginea. Depending on the time of their development and differentiation, the different cell types in the respiratory epithelium provide useful information on induced alterations because of the high degree of specialisation on their surface.