HIV infection of the CNS can result in neurologic dysfunction in a significant number of infected individuals. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence of HIV infection in neurons. Thus, neuronal damage and dropout are likely due to indirect effects of HIV infection of other CNS cells, through elaboration of inflammatory factors and neurotoxic viral proteins, including the viral transactivating protein tat. We and others demonstrated that tat induces apoptosis in differentiated mature human neurons. We now demonstrate that the high level of tat toxicity observed in human neurons involves specific developmental stages that correlate with N-methyl-D-aspartate receptor (NMDAR) expression, and that tat toxicity is also dependent upon the species being analyzed. Our results indicate that tat treatment of primary cultures of differentiated human neurons with significant amounts of NMDAR expression induces extensive apoptosis. In contrast, tat treatment induces only low levels of apoptosis in primary cultures of immature human neurons with low or minimal expression of NMDAR. In addition, tat treatment has minimal effect on rat hippocampal neurons in culture, despite their high expression of NMDAR. We propose that this difference may be due to low expression of the NR2A subunit. These findings are important for an understanding of the many differences among tissue culture systems and species used to study HIV-tat-mediated toxicity.