Glial cell-line-derived neurotropic factor (GDNF) and its receptors glial cell-line-derived neurotropic factor alpha (GFR1 alpha) and rearranged during transformation (R-ET) have been localized in the rat testis during postnatal development. The three mRNAs, and GDNF and GFR1 alpha proteins were detected in testis extracts from 1- to 90-day-old rats by reverse transcriptase PCR and Western blotting respectively. The three mRNAs were present in Sertoli cells from 20- and 55-day-old rats, pachytene spermatocytes (PS), and round spermatids (RS). The GDNF and GFRloc proteins were detected in PS, RS, and Sertoli cells. GDNF and GFR1 alpha were also detected using flow cytometry in spermatogonia and preleptotene spermatocytes, and in secondary spermatocytes. The localization of GDNF and GFR1 alpha in germ and Sertoli cells was confirmed by immunocytochemistry. The hypothesis that GDNF may control DNA synthesis of Sertoli cells and/or spermatogonia in the immature rat was addressed using cultures of seminiferous tubules from 7- to 8-day-old rats. Addition of GDNF for 48 h resulted in a twofold decrease in the percentage of spermatogonia able to duplicate DNA, whereas Sertoli cells were not affected. These results are consistent with a role of GDNF in inhibiting the S-phase entrance of a large subset of differentiated type A spermatogonia, together with an enhancing effect of the factor on a small population of undifferentiated (stem cells) spermatogonia. Moreover, the wide temporal and spatial expression of GDNF and its receptors in the rat testis suggest that it might act at several stages of spermato genesis.