T he targeted gene inactivation of endothelins-1 and -3 (ET-1 and ET-3) and of one of their receptors, ETB, in the mouse causes severe defects in the embryonic development. These defects, cardiovascular and craniofacial malformations for ET-1, and colonic agangliogenesis associated with skin pigmentation anomalies for ET-3 and the ETB receptor, reproduce pathological phenotypes due to natural mutations of the same genes in the mouse and the human. The mutant phenotypes have been causatively linked to deficient migration/proliferation/differentiation of neural crest cells, i.e., neurocristopathies. To bring new insight about the exact roles of ETs in development and the involvement of neural crest cells in these processes, we have explored, by in situ hybridization, the ontogeny in the early human embryo of the ET system (ET-1 and ET-3, ETA and ETB receptors, ET converting enzyme-1). ET receptor mRNA expression in neural crest cells starts at 3 wk of gestation and continues during the entire period studied (up to 6 wk of gestation). During this period, ETA expression progressively spreads to undifferentiated mesodermal components of various structures and organs (head and axial skeleton, lateral and ventral subdermal mesoderm), whereas ETB expression remains more restricted to fewer differentiated cells (neural tube, sensory and sympathetic ganglia, endothelium). Some of these tissues and structures that express either one of the receptors do not appear to be of neural crest origin. In the digestive tract and the cardiovascular area, the present observations on the sources of ETs and their target cells in the young embryo provide the basis for a dynamic interpretation of the results of gene targeting of the mouse and the human phenotypes, and point to other possible roles of ETs in other ontogenetic processes. The results support the concept of local, rather than hormonal, interactions between the sources and targets of ETs during development.