The Glutamatergic system is the most important excitatory system of the human brain. Dysfunction of this system plays an important role in the pathogenesis of numerous psychiatric disorders, including schizophrenia. According to the glutamatergic hypothesis of schizophrenia augmentation of the antipsychotic treatment with glycine - a coagonist of NMDA receptor helps to achieve better improvement. Aim. The aim of the study was to assess changes in mental status i.e. positive, negative and general psychopathology symptoms during augmentation of antipsychotic treatment with glycine in schizophrenic patients in a stable mental state with predominant negative symptoms. Methods. The study group consisted of 29 patients with a diagnosis of schizophrenia (ICD-10) with predominant negative symptoms. It was a 10-week prospective open-label study, which included 7 visits. Participants enrolled to the study received a fixed dose of antipsychotic drugs for at least 3 months prior to application of glycine. In addition to the existing antipsychotic treatment, patients were administered glycine orally (up to 60 g/day) for 6 weeks. 2 weeks before and after application of glycine, their mental status was also evaluated. At each visit we assessed severity of schizophrenia symptoms using PANSS. Results. After 6 weeks of glycine administration we observed statistically significant improvement in positive (PANSS P subscale, -7.8%, p<0.05), negative symptoms (N subscale, -16.1%, p<0.001), general psychopathology (G subscale, -12.2%, p <0.001) and PANSS total score (T, -12.8%, p <0.001). 2 weeks after the end of glycine augmentation mental status remained stable. Conclusion. In the course of treatment with glycine, moderate improvement in negative, positive and general psychopathology symptoms was observed. Use of glycine was safe and well tolerated. Side effects involved gastrointestinal complaints - mainly nausea and vomiting.