In order to assess the behavior of the spermatogonia that contribute to the stem cell system in steady-state mouse spermatogenesis, a series of pulse-labeling experiments of a subset of "undifferentiated spermatogonial" was performed using transgenic approaches. The results suggest that, in addition to the spermatogonial population that actually acts as the stem cells (actual stem cell compartment), a second set of undifferentiated spermatogonia also exists that possesses the potential to self-renew, but does not normally do so (potential stem cell compartment). The potential stem cells turn over rather rapidly, suggesting that they act as the transit amplifying cells in the normal situation. The potential stem cells seem to change their mode from transientamplification to self-renewal upon transplantation, regeneration, or loss of actual stem cells that occurs during the long reproduction period. Such a heterogeneous composition of the stem cell system may be important for the robustness of overall spermatogenesis.