Apoptosis of germ cells is very common in normal and injured mammalian testes. The aim of this study was to examine the possible involvement of the Fas and Fas ligand (FasL) system in the induction of germ cell apoptosis in normal and ischemia-reperfusion testes of adult mice. Apoptosis was assessed by the TUNEL method and by DNA gel electrophoresis. Fas and FasL mRNAs were detected by Northern blotting and reverse transcription polymerase chain reaction techniques, and proteins were analyzed by Western blotting and immunohistochemistry. Apoptosis of germ cells was identified in the normal testis especially around stages XI and XII, whereas the expression of Fas and FasL was largely confined to Leydig cells and Sertoli cells, respectively. However, in the testes reperfused after 1 h of ischemia, a high number of TUNEL-positive cells were identified in parallel with increased Fas-positive germ cells, whereas FasL expression in Sertoli cells was almost constant irrespective of the duration of reperfusion. Moreover, i.p. injection of anti-Fas antibody, which blocks the interaction between Fas and FasL, inhibited apoptosis, as indicated by the reduced number of TUNEL-positive cells, except for apoptosis at stages XI and XII. Our results indicate that the Fas/FasL system mediates apoptosis of spermatogenic cells in the injured testis but not spontaneous apoptosis in the normal testis.