Background: there is a two-way relationship between the gut microbiota (GM) and the brain, both in normal and pathological conditions. It has been suggested that disturbances in GM composition and function can lead to the activation of (neuro) inflammation or its maintenance in mental disorders. Objective: analysis of scientific publications, including the results of our own research on the gut microbiota, its role in the modulation of brain functions and involvement in the maintenance of (neuro)inflammation in endogenous mental disorders. Material and method: using the keywords “gut–microbiota” and “microbiota–gut–brain axis/mental disorders”, “inflammation”, “neuroinflammation”, articles and reviews were searched in databases and electronic libraries of scientific publications Medline/PubMed, Scopus, Google Scholar, RSCI, and other sources. Results: on the basis of the analysis of literature sources, general information about GM, its composition, functions and main ways of interaction with the brain is given. The role of GM in the development and maintenance of (neuro)inflammation, currently considered as one of the pathogenetic mechanisms of mental disorders, is discussed. Particular attention is paid to the results of own studies on the role of endotoxin (ET) (lipopolysaccharide — LPS) and inflammation in the development of endogenous psychoses and the relationship of these indicators with the effectiveness of pharmacotherapy. Conclusion: it has been established that GM is an important participant and regulator of neuro-immune interactions. The possible role of GM in the maintenance of (neuro)inflammation in endogenous mental disorders and the participation of endotoxin aggression (EA) in the formation of therapeutic resistance in endogenous psychoses were revealed.