Nitric oxide (NO) mediates glutamate neurotoxicity in primary cortical cultures. NO, first identified as endothelium-derived relaxing factor, is also a key neuronal messenger. The study shows that NO synthase (NOS) inhibitors, such as Nω-nitro-L-arginine (N-Arg) and Nω-monomethyl-L-arginine (Me-Arg), prevent neurotoxicity caused by N-methyl-D-aspartate (NMDA) and related excitatory amino acids. This effect is reversed by L-arginine, and depletion of arginine in the culture medium also reduces NMDA toxicity. Sodium nitroprusside, which releases NO, causes dose-dependent cell death, and hemoglobin, which binds NO, prevents neurotoxic effects of both NMDA and sodium nitroprusside. These findings confirm that NO mediates glutamate neurotoxicity. NO is produced from arginine in brain and endothelial cells by NOS, which has been purified and cloned. Macrophages and other blood cells also produce NO, which mediates their bactericidal and tumoricidal effects. However, the NOS in macrophages is a distinct protein from that in brain and endothelial tissues. Immunocytochemical studies have localized NOS to specific neuronal populations in the brain, retina, adrenal medulla, and intestine. Glutamate may mediate neurotoxicity in hypoxic-ischemic brain injury, and NMDA receptor antagonists prevent neuronal cell death in animal models. Glutamate neurotoxicity is also implicated in neurodegenerative disorders such as Alzheimer and Huntington diseases. NMDA receptor-mediated glutamate neurotoxicity involves calcium entry into cells via ligand-gated ion channels. The enhancement of NOS activity by NMDA stimulation of cerebellar slices also derives from calcium entry, which binds to calmodulin associated with NOS. The study used rat primary cortical cultures to demonstrate that glutamate neurotoxicity is prevented selectively by NOS inhibitors. The results show that NO is involved in NMDA neurotoxicity, as N-Arg reduces cell death, and L-Arg reverses this effect. Reduced hemoglobin, which binds NO, prevents NMDA-induced cell death. The study also shows that NO mediates cGMP formation, which is involved in NMDA-induced cell death. The study concludes that NO mediates glutamate neurotoxicity in neuronal cultures. NO is produced by NOS, and its activity is inhibited by NOS inhibitors. The study also shows that NO can cause cell death by reacting with superoxide to form peroxynitrite, a highly reactive molecule. The findings suggest that NO may have clinical implications in conditions involving neurotoxicity mediated through glutamate receptors.Nitric oxide (NO) mediates glutamate neurotoxicity in primary cortical cultures. NO, first identified as endothelium-derived relaxing factor, is also a key neuronal messenger. The study shows that NO synthase (NOS) inhibitors, such as Nω-nitro-L-arginine (N-Arg) and Nω-monomethyl-L-arginine (Me-Arg), prevent neurotoxicity caused by N-methyl-D-aspartate (NMDA) and related excitatory amino acids. This effect is reversed by L-arginine, and depletion of arginine in the culture medium also reduces NMDA toxicity. Sodium nitroprusside, which releases NO, causes dose-dependent cell death, and hemoglobin, which binds NO, prevents neurotoxic effects of both NMDA and sodium nitroprusside. These findings confirm that NO mediates glutamate neurotoxicity. NO is produced from arginine in brain and endothelial cells by NOS, which has been purified and cloned. Macrophages and other blood cells also produce NO, which mediates their bactericidal and tumoricidal effects. However, the NOS in macrophages is a distinct protein from that in brain and endothelial tissues. Immunocytochemical studies have localized NOS to specific neuronal populations in the brain, retina, adrenal medulla, and intestine. Glutamate may mediate neurotoxicity in hypoxic-ischemic brain injury, and NMDA receptor antagonists prevent neuronal cell death in animal models. Glutamate neurotoxicity is also implicated in neurodegenerative disorders such as Alzheimer and Huntington diseases. NMDA receptor-mediated glutamate neurotoxicity involves calcium entry into cells via ligand-gated ion channels. The enhancement of NOS activity by NMDA stimulation of cerebellar slices also derives from calcium entry, which binds to calmodulin associated with NOS. The study used rat primary cortical cultures to demonstrate that glutamate neurotoxicity is prevented selectively by NOS inhibitors. The results show that NO is involved in NMDA neurotoxicity, as N-Arg reduces cell death, and L-Arg reverses this effect. Reduced hemoglobin, which binds NO, prevents NMDA-induced cell death. The study also shows that NO mediates cGMP formation, which is involved in NMDA-induced cell death. The study concludes that NO mediates glutamate neurotoxicity in neuronal cultures. NO is produced by NOS, and its activity is inhibited by NOS inhibitors. The study also shows that NO can cause cell death by reacting with superoxide to form peroxynitrite, a highly reactive molecule. The findings suggest that NO may have clinical implications in conditions involving neurotoxicity mediated through glutamate receptors.