In a previous study we have shown that a subpopulation of primary sensory neurons contain brain-derived neurotrophic factor immunoreactivity. In the present study we investigated the distribution of brain-derived neurotrophic factor and its mRNA in cranial and spinal ganglia at different segmental levels, using immunohistochemical and quantitative reverse transcriptase-polymerase chain reaction techniques. Our results show that there is no significant difference in the percentage of brain-derived neurotrophic factor-immunoreactive neurons in spinal ganglia of different segmental levels. In contrast, more brain-derived neurotrophic factor- immunoreactive neurons were found in placode-derived than neural crest- derived ganglia. The percentage of brain-derived neurotrophic factor- immunoreactive neurons is consistent with the percentage of neurons lost after deletion of brain-derived neurotrophic factor or trkB genes. However, there is no correlation between brain-derived neurotrophic factor mRNA levels and the number of brain-derived neurotrophic factor immunoreactive neurons in these ganglia, suggesting that some neurons synthesize brain-derived neurotrophic factor while others accumulate the factor following its retrograde transport within nerve fibers. In particular, the proportion of brain-derived neurotrophic factor that is derived from extraganglionic sources in the placode-derived ganglia appears greater than that in the neural crest-derived ganglia.