The relationships between the nervous and the immune systems raise the question of whether neurotrophic factors, in addition to the regulation of neural cell ontogeny, may influence lymphocyte development, We report in this work that the pattern of neurotrophin receptor expression depends on the developmental stage of T cells, The presence of nerve growth factor receptor trkA could not be detected in any of the thymocyte subsets, whereas brain-derived neurotrophic factor (BDNF) receptor trkB was expressed in all thymocyte subpopulations. Interestingly, both trkB mRNA and protein expression inversely correlated with the maturation stage and the differentiation potential of thymocytes, being more expressed in CD4(-)8(-) immature thymocytes and progressively declining in CD8(+) and CD4(+) single-positive and CD4(+)8(+) more mature thymocytes, The developmentally regulated expression of trkB is further shown by the inhibition or enhancement of trkB expression induced by signals that either trigger or impair the transition from immature to more differentiated stages of the thymocyte developmental pathway, Signals generated following the interaction of BDNF with trkB receptor resulted in the stimulation of trkB autophosphorylation and in the up-regulation of the expression of the c-fos gene in CD4(-)8(-) cells and enhanced thymocyte survival, Finally, BDNF is expressed in thymic stroma and is further up-regulated by signals generated by the thymocyte/stromal cell interaction, These data suggest that BDNF may be a novel survival factor for thymocyte precursors and support the presence of developmentally regulated feedback mechanisms based on autocrine/paracrine neurotrophin/receptor interactions that may be involved in the thymocyte differentiation process.