Diabetes mellitus (DM) is a highly prevalent endocrine disease with a high risk of chronic complications. Damage to the central nervous system (CNS) is considered a serious DM complication. Diabetic encephalopathy (DE) is a specific CNS dysfunction that is characterized by impaired functioning of the brain. The root cause of DE may lie in a disrupted synthesis of various neurotransmitters. Impaired operation of the glutamatergic system is the key component of the pathophysiological mechanism responsible for the development of cerebral insufficiency in the setting of DM. Glutamine (Gln) is the main excitatory neurotransmitter of the CNS, which is involved in the processes of synaptic plasticity, learning and memory. Under physiological conditions, Gln concentrations must be kept at a minimum to ensure optimal operation of the brain. The activation of the glutamatergic system observed in DM is associated with neurotoxicity, leading to degeneration and death of neuronal cells. Excitotoxicity triggers the endoplasmic reticulum stress response, causes mitochondrial dysfunction and elevates oxidative stress. These are the three key pathophysiological mechanisms thought to underlie the development of DE. Oxidative stress is the most thoroughly studied of the pathological processes leading to DE, and is associated with damage to intracellular proteins, lipids and nucleic acids, resulting in the loss of neurons. Numerous preclinical and clinical studies have demonstrated the presence of a pathophysiological link between the activation of the glutamatergic system, excitotoxic mechanisms, and the development of DE. High levels of Gln were shown to correlate with deterioration of cognition, which intensifies with the course of the disease. Diagnosis and subsequent treatment of glutamatergic system dysfunction in patients with DM can be an important practical contribution to the minimization of clinical DE manifestations.