Excitotoxic neuronal death is mediated primarily by the N-methyl- d-aspartate receptor. N-methyl- d-aspartate induces two forms of excitotoxicity in CA1 pyramidal neurons of cultured rat hippocampal slices: the rapidly developing form that depends on external Na + and Cl −, and the delayed form that requires external Ca 2+ but not Cl −. Consistent with this notion, replacement of external Cl − with glucuronate, isethionate or methylsulfate attenuated or delayed selectively the rapid excitotoxicity. However, gluconate substituting for Cl − blocked both rapid and delayed forms of excitotoxicity. Gluconate also reduced N-methyl- d-asparatate-induced membrane currents recorded from CA1 neurons in a dose-dependent manner. This dose-dependence was remarkably similar to that observed for protecton of N-methyl- d-asparatate-induced neuronal death by gluconate. Although gluconate chelated Ca 2+ most strongly among the four Cl − substitutes examined, excitotoxic neuronal death could be protected by 7 mM gluconate without Ca 2+ chelating action. The voltage-dependent Mg 2+ block of N-methyl- d-aspartate receptors was not affected by gluconate. Gluconate suppressed the N-methyl- d-aspartate component of excitatory synaptic currents evoked in CA1 neurons. We conclude that protection of excitotoxic neuronal death by gluconate at low doses (<20 mM) is due to its antagonistic action on N-methyl- d-aspartate receptors. Gluconate is a widely used substitute for Cl −. Our unexpected findings give a warning that the results of any of the experiments concerning excitotoxicity or glutamate receptors obtained by gluconate substituting for Cl − must be interpreted with caution.