To address the possibility that stem cell factor (SCF) is a paracrine regulator of germ cell development in the adult rat testis, stage-specific distribution of SCF messenger RNA (mRNA) was investigated with Northern blot and in situ hybridization analyses. The highest levels of SCF mRNA were found in stages II-VI of the rat seminiferous epithelial cycle, whereas the lowest levels were in stages VII-VIII. Intermediate levels of SCF mRNA were detected in stages IX-XIV-I of the cycle. The expression of the SCF gene was found to be developmentally regulated, and the expression pattern followed the process of Sertoli cell proliferation and differentiation during postnatal life. The effect of mouse recombinant SCF on spermatogonial DNA synthesis was studied using an in vitro tissue culture system for stage-defined seminiferous tubules. A significant increase in DNA synthesis in spermatogonia could be detected when tubule segments from stage XII were cultured in the presence of 100 ng/ml SCF for 48 h (P < 0.05) and 72 h (P < 0.01). This observation was further confirmed with autoradiographic analyses; almost a 100-fold increase in thymidine incorporation in the SCF-treated (100 ng/ml) tubule segments was observed compared with that in untreated samples. The results of the present study suggest that SCF is a Sertoli cell-produced paracrine regulator and acts as a survival factor for spermatogonia in the adult rat seminiferous epithelium in a stage-specific manner.