In situ hybridizations using 35S-labelled antisense and sense RNA probes of rPLII, rPLP-A and rPLP-B were carried out on developing rat placenta to determine which cell types synthesized each specific mRNA. Cellular localization of the sites of synthesis of these placental RNAs would help to decide whether these proteins were functioning in the mother of the fetus. The cells of the basal zone are known to have access only to the maternal blood supply, while the labyrinth region is supplied by both maternal and fetal blood vessels. The data in this paper show that at day 12 of pregnancy the rPLII mRNA is synthesized in the primary and secondary giant cells. At later days, hybridization is seen in both the giant cells of the basal zone, and cells in the labyrinth, suggesting that rPLII has a function not only in the mother, but also in the fetus. The rPLP-A mRNA is synthesized in both the giant cells and the cytotrophoblasts of the basal zone. No hybridization is seen to any cells in the labyrinth, even at the later days when it appears that all cytotrophoblasts synthesize rPLP-A mRNA. The rPLP-B mRNA is synthesized exclusively by the cytophoblasts of the fetal placenta. Like rPLP-A, all these cells synthesize this mRNA in the late term placenta. The synthesis of the rPLP-A and rPLP-B mRNAs in cells which have access only to the maternal circulation suggest that they have a role in the mother.