The effects of HIV-1 Tat protein on mitochondria membrane permeability and apoptosis were analysed in lymphoid cells. In this report we show that stable-transfected HIV-Tat cells are primed to undergo apoptosis upon serum withdrawal. This effect was observed in both the Jhan T cell line and the K562 cells, the latter expressing the bcr-abl chimeric gene, which confers resistance to apoptosis induced by different stimuli. Using a cytofluorimetric approach we have determined that serum withdrawal induces a disruption of the transmembrane mitochondrial potential (Deltapsim) followed by an increase of reactive oxygen species (ROS) and the subsequent DNA nuclear loss in K562-Tat cells but not in the K562-pcDNA cell line. These pre-apoptotic events were associated with the cleavage of the caspase-3, while the expression of Bcl-2, Bcl-XL and Bax proteins was not affected by the presence of Tat. Regardless of the steady state of the Bax protein, we found that in both K562 and K562-Tat cells, this protein is located in the nucleus, but after serum withdrawal its localization was mainly in the cytoplasm. The activity of caspase-3 detected in K562-Tat cells after serum withdrawal paralleled with the mitochondria permeability transition. Nevertheless, in Jhan-Tat cells the inhibition of this caspase with the specific inhibitor, z-DEVD-cmk, did not affect the disruption of the mitochondria potential induced by serum withdrawal. Interestingly, we found that HIV-Tat protein accumulates at the mitochondria in the K562-Tat cells cultured under low serum conditions, and this mitochondrial localization correlated with the Deltapsim disruption detected in these cells. In addition, HIV-1 Tat protein synergies with protoporphyrin IX (PPIX), a ligand of the mitochondrial benzodiazepine receptor, in the induction of apoptosis in both Jhan and K562 cells. Thus, HIV-1 Tat protein may induce apoptosis by a mechanism that involves mitochondrial PT and may contribute to the lymphocyte depletion seen in AIDS patients.