The incorporation of ³H-proline by epithelial and connective tissue elements of rat palatal mucosae was studied in order to investigate the relative levels of protein synthesis by the epithelium and underlying connective tissue cells. Following a sixty minutes incorporation of the radioactive tracer in vitro, it was found that the suprabasal cells had most grains per unit area. Furthermore, the grains were more concentrated over the cytoplasm than the nucleus. This was in contrast with the labeling of basal cells which had twice as many grains over the nucleoplasm than that over the cytoplasm. In intermediate cells; i.e., the spinous layer, the number of silver grains per unit area was decreased from that of the suprabasal cells. In areas where desmosomes were more prominent, many grains were in touch with such desmosomes. However, the labeling appeared to be reduced as soon as the cells became flattened. Moreover, the epidermal keratohyalin granules were relatively free of grains. Except for certain intercellular surfaces the keratinized cells were generally free of grains. On the connective tissue side, silver grains were primarily localized over the fibroblasts with occasional grains being found over palatal muscle cells, neural elements and so on. Most grains over collagenous fibers were found in relation to mature collagen fibrils. Thus, protein synthesis in isolated mucosae of the rat palate appeared to take place both in epithelial and connective elements. There were no apparent tissue alterations caused by the in vitro incorporation procedure utilized under conditions of this study.