Palatal mucosa from neonatal rats was maintained under organ culture conditions in a chemically defined medium for periods up to 28 days. The histological state of the cultured palatal mucosa was compared with that of control tissue from growing animals of comparable age. The control tissues showed an increase in epithelial thickness, first noticeable at 17 days. Whilst the general structure of the tissues in organ culture was preserved for the duration of the experiment, some changes in epithelial behavior were evident. There was an increase in epithelial thickness up to 6 days, followed by a reduction in the nucleated cell layer of the epithelium to a thickness comparable with that at the start of the experiment. There was a loss of epithelial glycogen within the first day, with occasional reappearance of patchy and irregular deposits. Whereas the control of epithelial thickness appeared to be restored after 10 days in vitro, disturbances in the maturation of the keratinocytes, manifested as epithelial pearls and dyskeratotic cells, were evident at subsequent stages. Epiboly never occurred. The connective tissue component showed continued development, indicated by an increase in the thickness of collagen fibres. The overall palatal growth seen in vivo did not occur in organ culture. We suggest that the improved maintenance reported is partly the result of explanting tissues in such a way as to minimize trauma, and partly the result of incorporating serum albumin into the chemically defined medium.