Neurotrophins have profound effects on the development and maintenance of neurons that compose the VIIIth cranial nerve. In the auditory division of the nerve, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have been localized to the sensory epithelium, and their respective high-affinity tyrosine kinase receptors (TrkB and TrkC) are expressed within the neuronal population. By using a culture methodology that allows evaluation of single neurons, we determined that BDNF and neurotrophin-4 (NT-4), which both bind to the TrkB high-affinity receptor, greatly enhanced neuron survival above control cultures. NT-3, which acts via the TrkC high-affinity receptor, also increased survival, but to a lesser extent. By testing a variety of neurotrophin concentrations and combinations, we observed that simultaneous activation of the TrkB and TrkC receptors synergistically promoted neuron survival compared to cultures that contained either neurotrophin alone at the same total concentration. Antibody labeling showed that the high-affinity Trk receptors were localized predominantly to the neurons and not to the surrounding satellite cells; furthermore, TrkB- and TrkC-specific antibodies each labeled 100% of the cultured neurons. These results suggest that synergistic interactions between BDNF and NT-3 may be crucial for spiral ganglion neuron survival during the final stages of development. J. Comp. Neurol. 386:529–539, 1997. © 1997 Wiley-Liss, Inc.