Here we have studied the distribution of mRNA for tyrosine kinase B (trkB), the high-affinity receptor for brain-derived neurotrophic factor (BDNF) amongst serotonergic cell bodies of the raphe nuclei and their ascending projections into the dorsal hippocampus in the rat brain. Previous studies have shown that BDNF has got trophic action on serotonergic neurons. In the present study, we provide evidence that serotonergic neurons express mRNA for the functional receptor of BDNF, trkB. Intracerebro-ventricular (i.c.v.) injection of the 5-HT-specific neurotoxin, 5,7-dihydroxytryptamine, which lesions serotonergic cell bodies in the raphe nuclei as well as their ascending projections into the dorsal hippocampus, caused a dramatic loss of trkB mRNA from serotonergic cell bodies of the dorsal raphe nucleus. In contrast, there was no change in the abundance of trkB mRNA within the dorsal hippocampus. These findings provide direct evidence for the expression of trkB mRNA by serotonergic neurons and suggest distinct mechanisms of action of BDNF upon serotonergic neurons at the levels of their cell bodies and terminal projection sites.