The placenta synthesizes insulin-like growth factors (IGFs) and their binding proteins (IGFBPs), which are believed to regulate its growth and development in an autocrine/paracrine manner. To delineate the cellular sites of expression of IGP and IGFBP messenger ribonucleic acids (mRNAs) in human placenta throughout pregnancy, we used in situ hybridization histochemistry with 35S-labeled IGF and IGFBP complementary RNA probes on human placentas and fetal membranes of gestational ages 6 weeks to term (40 weeks). In placental regions where trophoblasts (fetal) or decidua (maternal) coexist (e.g. basal plate), the identity was delineated by their cytokeratin or vimentin immunoreactivity, respectively. Except for IGF-II, mRNAs encoding peptides of the IGF system were expressed in a similar spatial pattern and relative abundance throughout gestation. Both IGF mRNAs showed similar tissue distribution, but the IGF-II mRNA was more abundant than IGF-I mRNA at all gestational ages. IGF-II mRNA was expressed in the chorionic mesoderm of placental villi and chorionic plate in moderate abundance, and it decreased with gestation. It was also expressed in the trophoblasts of the cytotrophoblastic shell and Langhan's layer of placental villi only in the first trimester, suggesting an autocrine role for IGF-II in early cytotrophoblastic proliferation and/or differentiation. IGF-II mRNA was expressed most abundantly in the columns of intermediate trophoblasts in the anchoring villi and chorionic and basal plates. A gradient of IGF-II mRNA abundance was observed in the trophoblasts of the cytotrophoblastic column, with greater IGF-II mRNA levels in those at the invading front, suggesting a role for IGF-II in trophoblastic invasion. In the fetal membranes, IGF-II mRNA was identified in the amnion and chorion laeve. IGF-I receptor mRNA was expressed in low abundance in all cell types of the placenta. All six IGFBP mRNAs were identified in variable abundance in the decidualized stromal cells of the maternal decidua basalis and parietalis, with IGFBP-1 mRNA being expressed in the greatest abundance. The spatial pattern of expression of each IGFBP mRNA also differed among decidual cells, with IGFBP-1, IGFBP-2, IGFBP-4, and IGFBP-6 mRNAs being expressed in most cells, whereas IGFBP-3 and IGFBP-5 mRNAs were expressed in only some cells. IGFBP-1 mRNA was expressed initially in the epithelium of endometrial glands and in a population of decidualized stromal cells in early gestation, and subsequently in the majority of decidualized stromal cells. IGFBP-3 mRNA was expressed in both the decidua and certain intermediate trophoblasts of the basal plate and anchoring villi of placenta and in amnion and chorion laeve of fetal membranes. IGFBP-4 and IGFBP-5 mRNA were expressed additionally in low abundance in the chorionic mesoderm. IGFBP-6 was expressed in greater abundance in the decidua parietalis than in decidua basalis, although the general level of expression was low. The spatial pattern and relative abundance of expression of IGFBP mRNAs suggest IGFBP-1 to be the predominant IGFBP synthesized by the maternal decidual cells, interacting with the IGF-II that synthesizes fetal intermediate trophoblasts. Presumably, IGF-II and IGFBPs are used for cell to cell communication between fetal trophoblasts and maternal decidual cells at the feto-maternal interface for placental development and/or function.