We present evidence that, in quiescent cells, mitochondria deenergization is not correlated to apoptotic death; in fact thymocytes treated with oligomycin, a highly specific inhibitor of ATP synthase, alone or together with atractyloside to block ATP translocation from cytoplasm, were alive, even if they mitochondria were largely impaired, as revealed by flow cytometry. Furthermore, oligomycin was a powerful inhibitor of apoptosis induced in rat thymocytes by dexamethasone, calcium ionophore A23187 and etoposide, but was without effect when PCD was triggered by staurosporine and increased cell death in mitogen-treated thymocytes. The antiapoptotie effect of oligomycin in cells treated with A23187 or etoposide paralleled the inhibition of protein synthesis, while the inhibition of apoptosis induced by dexamethasone was strictly correlated to the effect exerted on cellular ATP level. Thymocyte apoptosis triggered by dexamethasone was blocked or delayed by inhibitors of respiratory chain, uncouplers, inhibitors of ATP formation, hypoxia and antioxidants without any correlation with their ability to reduce or increase oxygen radical formation in mitochondria. A lasting protection from dexamethasoneinduced apoptosis was always correlated to a drastic and rapid reduction of ATP level, while a delay in the death process was characterized by a moderate decrease of ATP. Oligomycin inhibited the specific binding of radioactive glucocorticoid to thymocyte nuclei, suggesting that ATP depletion blocks the primary event of glucocorticoid-induced apoptosis, completely inhibiting the onset of the apoptotic morphology. In conclusion, the reported data suggest that: 1) ATP may act as a cellular modulator of some forms of apoptosis depending on the cell cycle and on the death trigger; 2) mitochondria! function is not a general requirement of apoptosis; 3) the role of mitochondria in glucocorticoid-induced apoptosis may be more complex than the mere formation of reactive oxygen species.