System x_c⁻ exchanges intracellular glutamate for extracellular cystine, giving it a potential role in intracellular glutathione synthesis and nonvesicular glutamate release. We report that mice lacking the specific xCT subunit of system x_c⁻ (xCT^−/−) do not have a lower hippocampal glutathione content, increased oxidative stress or brain atrophy, nor exacerbated spatial reference memory deficits with aging. Together these results indicate that loss of system x_c⁻ does not induce oxidative stress in vivo. Young xCT^−/− mice did however display a spatial working memory deficit. Interestingly, we observed significantly lower extracellular hippocampal glutamate concentrations in xCT^−/− mice compared to wild-type littermates. Moreover, intrahippocampal perfusion with system x_c⁻ inhibitors lowered extracellular glutamate, whereas the system x_c⁻ activator N-acetylcysteine elevated extracellular glutamate in the rat hippocampus. This indicates that system x_c⁻ may be an interesting target for pathologies associated with excessive extracellular glutamate release in the hippocampus. Correspondingly, xCT deletion in mice elevated the threshold for limbic seizures and abolished the proconvulsive effects of N-acetylcysteine. These novel findings sustain that system x_c⁻ is an important source of extracellular glutamate in the hippocampus. System x_c⁻ is required for optimal spatial working memory, but its inactivation is clearly beneficial to decrease susceptibility for limbic epileptic seizures.