In our study, patients with schizophrenia will be randomly assigned to receive either NAC or placebo for 8 weeks. Brain levels of glutamate and an important antioxidant, glutathione, will be measured before and after treatment, using a neuroimaging technique known as magnetic resonance spectroscopy. Cognitive and negative symptoms will also be assessed before, during and after treatment. The investigators hypothesize that glutamate and glutathione will be normalized in patients' brains, and that their negative and cognitive symptoms will be improved, too.