The present study investigates the molecular apoptotic pathway in germ cells following acute ischemia of the rat testis. Rats were subjected to ischemia-inducing torsion and testes were harvested after reperfusion. Apoptotic cells were identified with an antibody to single-stranded DNA. Seminiferous tubule RNA was examined by RNase protection assay or by reverse transcriptase-polymerase chain reaction (RT-PCR) for the presence and regulation of apoptotic molecules. Proteins from seminiferous tubules were used for Western blot analysis of cytochrome c. Germ cell apoptosis was maximal at 24 h after repair of torsion. Germ cells in stages II-III of the seminiferous epithelium cycle were the predominant early responders. The RNase protection assays revealed that Bcl-X(L) was the prominent mRNA species. Caspases 1, 2, 3, and Bax mRNA were consistently upregulated; however, the time of upregulation after torsion was variable. The Bcl-X(L) and Bcl-X(S) mRNAs were less consistently upregulated and there was no evidence for upregulation of Fas or Bcl-2. Fas ligand (FasL) was not detected by RNase protection assay, but RT-PCR revealed a significant increase in FasL expression 4 h after the repair of torsion. Western blot analysis for cytochrome c release demonstrated a significant increase 4 h after the repair of torsion. Results suggest that germ cell apoptosis following ischemia/reperfusion of the rat testis is initiated through the mitochondria-associated molecule Bax as well as Fas-FasL interactions.