In order to establish whether the neurotoxicity of the human immunodeficiency virus type 1 (HIV-1) regulatory protein Tat could be related to the production of potentially toxic substances by microglial cells, we examined the ability of recombinant HIV-1 Tat protein to stimulate the release of NO in purified rat microglial cultures. We found that the exposure of microglia to Tat led to a dose dependent expression of the inducible isoform of nitric oxide (iNOS) and NO production. The effect was remarkably enhanced by pretreatment or cotreatment with the proinflammatory cytokine interferon-gamma (IFN-gamma), but not with bacterial lipopolysaccharide (LPS). The high concentrations of Tat required (>100 ng/ml) suggested the viral protein induced transactivation of the iNOS gene, rather than acting through a receptor-mediated mechanism, that generally requires lower concentrations. Indeed, the induction of the iNOS gene by Tat was largely prevented by a specific inhibitor of the nuclear factor-kB (NF-kB), a transcription factor known to be involved in the induction of iNOS by LPS. The activation of NF-kB could largely account for the ability of Tat to induce iNOS expression and to act in synergism with IFN-gamma, which utilizes a different transduction system. On the other hand, the convergence of Tat and LPS on the same target (NF-kB) could explain the lack of synergism between these substances. We propose that the induction of iNOS in microglial cells and the consequent release of high and sustained levels of NO during HIV-1 cerebral infection may be an important step in the cascade of pathological events triggered by Tat. Furthermore, the NO-dependent damage may be exacerbated by the presence of IFN-gamma, which is likely to occur in pathological conditions characterized by glial activation and inflammatory cell infiltration.