Two receptor tyrosine kinases, ErB-3 and ErbB-4, mediate signaling by Neu differentiation factors (NDFs, also called neuregulins), while ErbB-1 and ErbB-2 serve as co-receptors. We show that the two NDF/neuregulin receptors differ in spatial and temporal expression patterns: The kinase-defective receptor, ErbB-3, is expressed primarily in epithelial layers of various organs, in the peripheral nervous system, and in adult brain, whereas ErbB-4 is restricted to the developing central nervous system and to the embryonic heart. An example of alternating expression of the two receptors is provided by the developing cerebellum: During postnatal cerebellar development, ErbB-4 expression slightly decreases along with a decline in NDF transcription, whereas ErbB-3 expression commences after the peak of neurogenesis. To study functional differences, we established primary brain cultures and found that ErbB-3 was expressed only in oligodendrocytes, whereas ErbB-4 expression was shared by oligodendrocytes, astrocytes and neurons. Blocking the action of endogenous NDF in vitro, by using a soluble form of ErbB-4, accelerated neurite outgrowth in both primary cultures and in neuronal-type cultures of the P19 teratocarcinoma, suggesting an inhibitory effect of NDF on neural differentiation. Apparently, ErbB-3 is associated with proliferation of P19 cells, whereas ErbB-4 correlates with a differentiated phenotype. We conclude that the two NDF receptors play distinct, rather than redundant, developmental and physiological roles.