The expression of stem cell factor (SCF) mRNA and protein was examined in the postnatal rat testis. Northern blot analysis of total RNA from 1-4 days postpartum (dpp) testes showed none or barely detectable levels of the approximately 6.5 kb SCF transcript. At 5 dpp, there was a striking elevation in this mRNA, a timing that coincides with the onset of spermatogonial proliferation. Immunohistochemical staining of testes showed that SCF protein was readily detected within Sertoli cells at 1-7 dpp and in the adult. Immunoreactive material was also detected within gonocytes and within Leydig cells at 1-7 dpp and in Leydig cells in the adult. Given the intense staining of Leydig cells by two different antibodies to SCF, the potential synthesis of SCF mRNA by these cells was investigated. Northern blot and reverse transcription/polymerase chain reaction analysis indicated that adult Leydig cells do not synthesize SCF mRNA, and Sertoli cells do. The significance of these findings is discussed in the context of germ cell-Sertoli cell interactions and Leydig cell function.