Hie present review aspires to summarize the major advancements in the experimental study and understanding of the internal regulation of the intensity of rhizobial symbiosis. This kind of symbiosis takes place in the roots of predominantly legume plants in the form of specialized organs, root nodules, that are colonized by the soil nodule bacteria (rhizobia). Although the bacterial microsymbionts belong to several taxonomic groups, they share the ability of initiating root nodule development and fixation of atmospheric dinitrogen, which is subsequently utilized by the host plant The latter feature predetermines the economic importance of rhizobial symbiosis and of the regulation of its intensity. While the symbiotic nodule induction is mediated by highly specific bacterial lipochitooligosaccharides, called Nod factors, further regulation of plant response is strictly controlled by plant endogenous mechanisms. Surprisingly, the major regulatory circuit includes systemic signaling from the root to the shoot and backwards. The nature of the involved molecules has been unknown until recently, except for the shoot-acting protein HAR1, described in 2003, and its homologs. Starting from 2008, a number of additional shoot-acting or root-acting regulatory mutants have been isolated. Their ongoing characterization has identified a series of new genes and products involved in the systemic signal generation, transduction and perception. Genomic approach allowed to identify low-molecular weight peptides of the CLE family as symbiotic signal messengers. The new experimental data enable a preliminary reconstruction of the systemic circuit and prediction of the nature of yet unknown components. A special interest represents the mechanism by which the inhibitory signal of ambient nitrate is integrated into the symbiotic circuit.