The localized developmental expression of murine platelet-derived growth factor A (PDGF-A) was compared to that of its receptor (Pdgfra). Our in situ hybridization study included germ layers of primitive streak embryos, early axial structures (dermatome, myotome, sclerotome, floor plate), the skin and some of its derivatives (hair and mammary gland), the developing forelimb, the branchial arches and various sense organs (otic vesicle, olfactory epithelium and the eye). We report that PDGF-A and Pdgfra are expressed in separate, but adjacent cell layers in these structures and that in most, the ligand is expressed in the epithelium, whereas the receptor in the mesenchyme. This localization corresponds to classical experimental evidence for developmental interactions across cell layers. We suggest that the spatio-temporal regulation of PDGF-A and Pdgfra, and other related systems, represents one model for the spatial regulation of receptor-ligand interactions.