Quantitative in situ hybridization and northern blot analysis techniques were used to determine the topographical distribution and levels of mRNA coding for brain-derived neurotrophic factor (BDNF) and the tyrosine receptor kinase (trkB) mRNA in the hippocampal formation of two strains of male rat during aging. Age did not change the prevalence or regional distribution patterns of BDNF or trkB mRNA in the hippocampal formation throughout the lifespan of male Sprague-Dawley rats. There also were no significant differences in the prevalence or topographical distribution patterns of trkB mRNA transcripts during aging. Northern blot analysis of hippocampal RNA from male Fischer 344 confirmed that neither BDNF mRNA nor trkB mRNA levels changed with age. These findings suggest that age-related neurodegenerative changes, including the atrophy of the cholinergic septo-hippocampal pathway, are not the result of changes in hippocampal BDNF or trkB mRNA expression. Moreover, the prevalence and distribution of synaptosomal-associated protein 25 (SNAP-25), a neuron-specific protein located in synaptic terminals and a putative marker of synaptic integrity, did not change with age. These findings indicate that altered hippocampal synaptic plasticity which occurs in the aged rat brain is not a reflection of changes in the expression of BDNF or trkB receptor mRNA.