The morphological changes induced in rat germ cell mitochondria by chlorobenzyl-1 H-indazol-3-carboxylic acid (AF 1312/TS) and lonidamine (AF 1980), two antispermatogenic compounds, are described. Twenty-four hours after treatment, numerous dumbbell-shaped mitochondria appear in the Golgi, cap, and acrosomic spermatids, whereas after 48 hrs severe signs of mitochondrial degeneration are visible in the maturation phase spermatids. Biochemical studies performed on isolated germ cells and their mitochondria, harvested from the testes of normal and treated rats, are also described. In the normal rats, the cells and mitochondria were incubated with the antispermatogenic compounds. Aerobic and anaerobic glycolysis was evaluated on isolated germ cells, whereas the following parameters were evaluated in the mitochondria: ADP O ratio, respiratory control ratio (RCR), and ADP affinity. The results obtained demonstrate that the antispermatogenic agents induce, both in vivo and in vitro, similar changes in energy metabolism; respiration, RCR, and ADP affinity are significantly reduced, while aerobic glycolysis is increased.