We have investigated the molecular nature of the high affinity nerve growth factor (NGF) receptors by using cell lines expressing gp75LNGFR and gp140trk. Our results suggest that gp75LNGFR and gp140trk interact with NGF independently and that only gp140trk mediates NGF signaling. NGF binds to gp140trk with picomolar affinity and induces its phosphorylation on tyrosine residues regardless of the presence of gp75LNGFR. NGF-gp140trk complexes display the slow dissociation rate and rapid internalization characteristics of high affinity NGF receptors. Cross-linking studies reveal the existence of gp75LNGFR and gp140trk homodimers. However, we were unable to detect gp75LNGFR-gp140trk heterodimers. Coexpression in COS cells of wild-type and kinase deficient mutants reveals that gp140trk receptors can undergo intermolecular phosphorylation, indicating the formation of functional homodimers. Moreover, these kinase deficient mutants inhibit NGF-induced signaling through wild-type gp140trk receptors. These results indicate that the functional high affinity NGF receptors consist of gp140trk homodimeric (or oligomeric) complexes.