Cyanobacteria synthesize neurotoxic ß-N-methylamino-L-alanine (BMAA). The roles of this non-protein amino acid in cyanobacterial cells are insufficiently studied. Currently there are only two published studies on the impact BMAA cause on dizotrophic cyanobacteria. During diazotrophic growth, filamentous cyanobacteria form single differentiated cells, called heterocysts, which are separated by approximately twelve-fifteen vegetative cells. It was found out that that exogenous BMAA decreases the enzyme activity of nitrogenase in heterocysts of a dizotrophic strain Anabaena (Nostoc) sp. PCC 7120 [Berntzon et al., 2013; Popova et al., 2018]. Moreover, we have discovered that this cyanotoxin inhibits the heterocyst formation in filamentous cyanobacteria under nitrogen deprivation [Popova et al., 2018]. To learn the reason behind the inhibition of heterocyst formation we applied the quantitative PCR and tested the transcriptional activity of several genes that control cell differentiation in Anabaena sp. PCC 7120 during nitrogen step-down. We discovered that addition of exogenous BMAA leads to down-regulation of hetR and hepA genes in Anabena sp. PCC 7120 [Popova et al., 2018]. When combined nitrogen is available heterocyst formation is blocked and cyanobacterial filaments contain only vegetative cells. In the present study, we discovered that exogenous BMAA induced the processes leading to heterocyst formation in filamentous cyanobacteria under repressive conditions. BMAA treated cyanobacteria form heterocyst-like dark non-fluorescent non-functional cells in the growth medium containing nitrogen. The BMAA mediated derepression is eliminated by glutamate. The quantitative PCR permitted to detect the BMAA impact on the transcriptional activity of several genes implicated in nitrogen assimilation and heterocyst formation under nitrogen-replete conditions in Anabaena sp. PCC 7120. We demonstrated that expression of several essential genes increased in the BMAA presence under repressive conditions. Data obtained are important for further fundamental studies of regulatory role of cyanobacterial secondary metabolites.