Acquired immunodeficiency syndrome (AIDS) is frequently associated with dementia. The wide spectrum of neurological abnormalities associated with this dementia may involve a neurotoxin that activates the NMDA subtype of glutamate receptor in neurons. We have found that the antimicrobial agent pentamidine, which is prescribed for AIDS patients for the prophylaxis and treatment of Pneumocystis carinii pneumonia, is an effective NMDA receptor antagonist. Pentamidine inhibited 3H- dizocilpine binding to the NMDA receptor in rat brain membranes at a site separate from glutamate, glycine, and spermidine, with an affinity near 2 microM. Similar concentrations of pentamidine block NMDA-induced increases in intracellular Ca2+ and NMDA-induced currents in cultured forebrain and cortical neurons, apparently without use dependence or voltage dependence, suggesting that pentamidine may represent a novel chemical class of NMDA receptor antagonist. Finally, pentamidine protects neurons from the lethal effects of acute NMDA exposure in vitro. AS pentamidine may accumulate in the brain at relevant concentrations following repeated high-dose parenteral administration, these findings suggest that the drug may be neuroprotective in vivo.