A large part of germ cells die apoptotically during testicular development in rodents. In the present study, a wave of germ cell apoptosis was observed between days 10 and 30 of postnatal life by in situ 3'-end labeling and DNA fragmentation analysis. To explore the potential involvement of Bcl-2 family members in this process, the expression and localization of some Bcl-2 family proteins (Bcl-2, Bcl-xL, Bcl-w, Bak, Bax, and Bad) and p53 were analyzed during testicular development in the rat by Western blotting and immunohistochemistry. The dynamic changes in the expression profiles of Bcl-2 family proteins are consistent with a model in which germ cells are primed for apoptosis during the first cycle of spermatogenesis by de novo expression of the death effectors Bax and Bad in a p53-dependent manner and these proteins are prevented from triggering further apoptosis after the first spermatogenic cycle has been set up by anti-apoptotic Bcl-2 family proteins Bcl-xL and Bcl-w. To examine whether the pro-survival effect of stem cell factor (SCF) on germ cells in vitro is mediated by Bcl-2 family proteins, the correlation between the pro-survival effect of SCF on germ cells and the expression of the above-mentioned apoptosis-related gene products in the seminiferous tubules at stage XII of the epithelial cycle were also investigated using a tubular culture system. The data suggest that SCF supports germ cell survival during spermatogenesis by up-regulating pro-survival Bcl-2 family proteins, Bcl-w and Bcl-xL, and down-regulating pro-apoptosis Bcl-2 family proteins, e.g. Bax.