The TrkB and TrkC receptor tyrosine kinases have been identified as high-affinity receptors for the neurotrophic factors brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5) and NT-3 respectively. These receptor classes were identified and mapped by the in situ hybridization of antisense riboprobes complementary to portions of the intracellular (tyrosine kinase) or extracellular (ligand-binding) domains of trkB and trkC mRNA, and by the distribution of high-affinity [125I]BDNF, [125I]NT-4/5 and [125I]NT-3 binding sites in adjacent rat brain sections. Both methods showed that TrkB and TrkC receptors are abundant and widely expressed throughout the brain. Kinase or extracellular domain trkC probes labelled neuronal somata in a qualitatively similar manner in virtually every major area of the forebrain. Neither trkC probe labelled non-neuronal cells except for elements within cerebral arteries and arterioles. The kinase domain trkB probe hybridized exclusively to neurons. Neurons expressing trkB were even more widely distributed than those expressing trkC. The extracellular domain trkB probe labelled neurons with the same relative distribution as the trkB kinase domain probe, but also hybridized extensively with non-neural cells, particularly astrocytes, ependyma and choroid epithelium cells. The distribution of [125I]NT-3 binding sites generally resembled that of trkC hybridization, particularly in the neocortex, striatum and thalamus. [125I]BDNF and [125I]NT-4/5 binding sites were more widely distributed and denser than those for [125I]NT-3, and resembled the trkB hybridization pattern. These patterns are consistent with the preferential binding in the brain of TrkC receptors by [125I]NT-3 and of TrkB receptors by [125I]BDNF and [125I]NT-4/5. That the predominantly neuronal patterns of hybridization obtained with kinase and extracellular domain probes for trkC are qualitatively indistinguishable suggests that truncated and full-length forms of TrkC are expressed within extensively overlapping populations of neurons. In marked contrast to TrkC, expression of the full-length and truncated forms of TrkB appears to be largely segregated, being expressed principally on neurons and non-neuronal cells respectively. The abundant and widespread neuronal distribution of full-length, signal-transducing forms of TrkB and TrkC predict that their cognate ligands, BDNF, NT-4/5 and NT-3, may exert direct effects on a large proportion of neurons within the mature brain.