Insulin-like growth factor I(IGF-I) stimulates the growth of a variety of cell types in vivo and in vitro, and its plasma concentration is closely regulated by growth hormone. Histochemical studies on the distribution of IGF-I immunoreactivity and IGF-ImRNA in adult and fetal tissues of several species have also shown multiple sites for the synthesis of IGF-I, implying a paracrine/autocrine as well as an endocrine mechanism for the action of this growth factor. In order to clarify the role of IGF in process of morphogenisis, the expression of mRNA for IGF-I receptor (IGF-IR) was investigated in the submandibular gland of developing rats by in situ hybridization technique. The expression of the IGF-IR mRNA signal was observed in the cell of terminal portion and in the duct at newborn. The signal in ductal cell at newborn was stronger than that in the cell of terminal portion. Beside, the signal of IGF-IR mRNA was shown in the endothelial cell around the ductal system. With the differentiation of submandibular gland, the signal of IGF-IR mRNA was shown negatively with aciner cells, however, those signal were observed at undifferentiat cells in acinus. In contrast, although the ductal system was identified as intercalated ducts, striated ducts and excretory ducts at postnatal 21 days, the signal of IGF-IR mRNA had been observed continuously since newborn stage. At postnatal 50 days, the IGF-IR mRNA signal localized exclusively in the granular convoluted tubules and intercalated ducts, but it was not observed in acinus. These results suggest that IGF-IR induces proliferation and growth of cells in early stage of submandibular gland development. In addition, the expression of IGF-IR mRNA signal may have an important role in the differentiation into aciner cells and granular convoluted tubule cells in morphogenisis of the rat submandibular gland.