An actual biological problem is to identify the effect of mycorrhization by fungi of arbuscular mycorrhiza (АМ) on the metabolic profiles of plant roots at the key stages of host plant development. The aim of the study was to evaluate the effect of Rhizophagus irre-gularis inoculation and/or phosphorus treatment on the root metabolome of Medicago lupulina at the stages from the first leaf to the mature fruit. The assessment of metabolic profiles was performed using GC-MS. 327 metabolites were identified: 26 amino acids, 20 carboxylic acids, 14 fatty acids, 58 sugars, sterols, phenolic compounds, etc. The efficient AM is characterized with upregulation of the metabolism of proteins, carbohydrates and lipids, as well as an increase in the content of phosphates. The level of acids of tricar-boxylic acid cycle were generally lower during mycorrhization. For the first time, the lateral branching initiation stage was shown to have a key importance for the development of efficient AM symbiosis. 14 metabolic markers of the efficient AM symbiosis development were identified. The conclusion is that AM significantly improve plant adaptation to low Pi conditions through metabolic alterations.