Apoptosis of neurons and non-neuronal cells has been demonstrated in the brain of AIDS patients with dementia. Previous studies suggest that the apoptotic stimuli are likely to be soluble factors. Several candidates for the soluble factors that lead to neuronal apoptosis in HIV-1 infection have been proposed, including the HIV-1 Tat protein and TNF-alpha. The mechanisms that lead to neuronal apoptosis in the brain of AIDS patients in vivo, may involve the combined effects of more than one pro-apoptotic factor. In this study, we examine whether exposure of primary human neurons to the combination of HIV-1 Tat and TNF-alpha can potentiate the induction of neuronal apoptosis compared with exposure to either factor alone. TNF-alpha was shown to potentiate the induction of neuronal apoptosis by HIV-1 Tat via a mechanism that involves increased oxidative stress. Antioxidants inhibited, but did not completely abolish the induction of neuronal apoptosis by Tat, suggesting that other mechanisms are also likely to be involved. These findings suggest that soluble HIV-1 Tat and TNF-alpha may play a role in neuronal apoptosis induced by HIV-1 infection of the CNS, particularly when present in combination. Our findings further suggest that one mechanism whereby combinations of pro-apoptotic factors may potentiate the induction of neuronal apoptosis in the brain of AIDS patients is by increasing oxidative stress. Understanding the role of oxidative stress and other mechanisms that lead to apoptosis in HIV-1 infection of the CNS may advance the development of new therapeutic strategies to prevent neuronal cell death and improve neurologic function in AIDS patients.