The understanding of testicular physiology, pathology, and male fertility issues requires knowledge of male germ cell death and energy production. Here, we induced human male germ cell apoptosis (detected by Southern blot analysis of DNA fragmentation, TUNEL, activation of caspases-3 and -9, and electron microscopy) by incubating seminiferous tubule segments under hormone-and serum-free conditions. Inhibitors of complexes I to IV of mitochondrial respiration, exposure to anoxia, and inhibition of F0F1-ATPase (with oligomycin) decreased the ATP levels (analyzed by HPLC) and suppressed apoptosis at 4 h. Uncoupler 2,4-dinitrophenol (DNP) and oligomycin combination also suppressed death at 4 h, as did the DNP alone. Inhibition of glycolysis by 2-deoxyglucose neither suppressed nor further induced apoptosis nor altered the antiapoptotic effects of the mitochondrial inhibitors. Furthermore, Fas system activation did not modify the effects of mitochondrial modulators. After 24 h, delayed male germ cell apoptosis was observed despite the presence of the mitochondrial inhibitors. We conclude that the mitochondrial ATP production machinery plays an important role in regulating in vitroinduced primary pathways of human male germ apoptosis. The ATP synthesized by the F0F1-ATPase seems to be the crucial death regulator, rather than any of the complexes (I-IV) alone, the functional electron transport chain, or the membrane potential. We also conclude that there seem to be secondary pathways of human testicular cell apoptosis that do not require mitochondrial ATP production. The present study emphasizes the role of the main catabolic pathways in the complex network of regulating events of male germ cell life and death.testis; spermatogenesis; apoptosis; mitochondria; oxidative phosphorylation THE ENERGY METABOLISM and catabolism in the testis involve a unique network of reactions and include several testis-specific enzymes, hormonal regulation, and essential cell-to-cell interactions (1,8,27,29,41,43,55,56,58). The proper functioning of this network is critical for testicular physiology. Another crucial regulator of normal testicular function is appropriate germ cell death, and the disruption of this orderly process is associated with several male reproductive disorders (15,16,30,38,47,54,60). How these two entities, i.e., energy metabolism/catabolism and male germ cell death, are linked is presently unclear.The cell types of the seminiferous epithelium differ from each other in their sensitivity to death-inducing signals and with their preferred substrates for energy metabolism. Spermatogonia, mature spermatozoa, and the somatic Sertoli cells exhibit high glycolytic activity, whereas spermatocytes and spermatids produce ATP mainly by mitochondrial oxidative phosphorylation (OXPHOS; see Refs. 3,28,31,37,45,53,64). Interestingly, the cell types that use OXPHOS for energy production (i.e., spermatids and spermatocytes) are sensitive to death-inducing signals such as hormonal deprivation, Fas activation, and elevated temperature ...