The avian thymus and bursa of Fabricius are the specific organs where the maturation and differentiation of T- and B-lymphocytes, respectively, take place. In the mammalian lymphoid organs mRNAs of the neurotrophins and their receptors have been identified but their localization at the protein level remains still unknown. This study was undertaken to analyze the localization of the Trk family of tyrosine kinase receptors in the avian primary lymphoid organs (thymus and bursa of Fabricius) during the posthatching development using immunohistochemistry. These proteins serve as essential constituents of the high affinity receptors for neurotrophins. In the thymus of all groups of age specific immunoreactivity (IR) was observed for all three Trks: TrkA-like IR was found labelling medullary epithelial cells and a subpopulation of cortical epithelial cells; TrkB-like IR was found in the medullar dendritic cells and cortical macrophages; TrkC-like IR labelled the cortical epithelial cells and scattered medullar clusters of epithelial cells (including Hassal's corpuscles). Quantitative analysis revealed age-dependent decrease in the area occupied by TrkA-like IR in the cortex, and age-dependent increase in the medulla; no changes were detected in the area occupied by TrkB-like IR; the TrkC-like immunoreactive cells increase from 7 to 30 days and then decrease. Regarding to the bursa of Fabricius, TrkA-and TrkC-like IR were exclusively found in the epithelial cells of the follicle associated and the interfollicular epithelia, as well as TrkC-like IR in some medullary reticular epithelial cells of adult animals. Nevertheless, TrkB-like IR labelled extrafollicular unidentified cells in 7 days old animals, and the follicular secretory dendritic cells at 30 and 60 post-hatching. The area occupied by the medullary TrkB-like IR cells increased between 30 and 60 days. No immunostaining of lymphocytes was observed for any of the assessed antigens. The blood vessels of both the thymus and the bursa of Fabricius were immunoreactive for TrkA- and TrkC-like proteins. The present results provide evidence for the localization of Trks in the non-lymphoid cells (epithelial and dendritic) of the avian primary lymphoid organs, suggesting a role for neurotrophins in these cells. Moreover, the selective cell localization of each Trk protein, and the absence of apparent overlapping, claims for a differential role of the specific Trk ligands. Whether or not these findings have functional relevance for T- and B-lymphocytes processing in avian primary lymphoid organs is discussed.