The reaction of the soybean symbiotic apparatus inoculated with Bradyrhizobium japonicum strains and Tn5 mutants, which were different in efficiency, was studied for the loss of water content in leaves and roots induced by prolonged drought, as well as the seed productivity of the formed symbiotic systems. To conduct researches were used microbiological, physiological, biochemical methods, gas chromatography and spectrophotometry. The objects of the study were selected symbiotic systems formed with the participation of soybean plants and strains B. japonicum 646 (active, virulent) and 604k (inactive, highly virulent), as well as Tn5-mutants – B1-20 (active, virulent) and 107 (low-active, virulent) obtained by the method of transposon mutagenesis in the department of symbiotic nitrogen fixation at the Institute of Plant Physiology and Genetics of the National Academy of Sciences of Ukraine. Before sowing, sterilized with 70 % ethanol and washed under running water for 1 h, the seeds were inoculated with suspensions of nodules bacteria (the titre of the suspension was 108 cells in 1 ml). The combined model drought was created during 12 days by stopping watering of plants up to 40 % of full moisture content starting from the stage of two true leaves and gradual transfer of watering to 30 % of full moisture content in the stage of three true leaves and budding - the beginning of flowering. After the stopping of drought, the moisture content of the substrate was adjusted to 60 % of full moisture content (watering recovery) into the stage of mass flowering. Control plants were inoculated by rhizobium culture, as well as plants without inoculation, which grew for optimal watering. It was investigated that in symbiotic systems formed with the participation of soybean and the active strain B. japonicum (646) and Tn5-mutant (B1-20) there was no significant reduction in the water content of plants under drought conditions and the effective work of the symbiotic apparatus was recorded, which contributed to the preservation of seed productivity. Ineffective symbiotic systems observed significant losses in water content and inhibition of the process of nodulation (strain 604k) and nitrogen fixation (Tn5-mutants 107), which was accompanied by significant losses of soybean crop yields. As a result of the research, it was concluded that in soil-climatic conditions with insufficient rainfall and frequent droughts, effective symbiotic systems should be used, which will promote the optimal functioning of the symbiotic apparatus and preserve the seed productivity of soybeans by adaptive regulation of water balance and fixation of molecular nitrogen of the atmosphere. The study of the functioning of leguminous plants in symbiosis with strains of nodule bacteria is important for finding effective symbiotic systems that are able to realize their adaptive potential for the effects of stress factors, in particular drought. Effective symbiotic relationships are the main source of nitrogen fixation in terrestrial ecosystems, which will reduce the need to enrich the soil with chemical compounds and provide additional economic and environmental advantage.