Leukemia inhibitory factor (LIF), a pleiotropic cytokine, is expressed in the rat testis and produced predominantly by peritubular myoid cells. The aims of this study were to characterize the testicular cell targets of LIF and to identify the role of LIF in the testis. The LIF receptor (LIF-R)/gp190 transcript was detected by reverse transcription-polymerase chain reaction (RT-PCR) in the rat testis from Day 13.5 postcoitum until adulthood. Seven highly purified testicular cell populations, representative of the major testicular constituents, were studied at transcriptional and protein levels by, respectively, RT-PCR and flow cytometry with biotinylated-LIF. Spermatogonia and, to a lesser extent, the somatic cells, exhibited specific LIF-binding sites. These results were strengthened by in situ analysis, showing predominant LIF-R immunoreactivity in spermatogonia at all ages studied. In addition to the 190-kDa LIF-R, Western blot analysis revealed the presence of a 50- to 60-kDa C-terminal gp190 isoform. This truncated form, which is unable to bind LIF, was the only form expressed in meiotic germ cells, suggesting an original down-regulation process of LIF-R expression during spermatogenesis. Finally, we showed that LIF increased [³H]-thymidine incorporation in spermatogonia in microdissected, cultured seminiferous tubules. Taken together, our results strongly suggest that LIF has a role in the regulation of the spermatogonial cell compartment.