Low intracellular K+ concentration ([K+]i) promotes apoptosis and blocking K+ loss prevents apoptosis, but the mechanism of action of low [K+]i remains unclear. Here, we show that low [K+]i increases NF-kappaB transcriptional activity by enhancing its binding to the promoter of target genes without affecting its activation and nuclear translocation in cortical neurons deprived of serum. Low K+ concentration promotes NF-kappaB/DNA binding through direct effects on the interaction of NF-kappaB dimers with DNA. Up-regulation of proapoptotic protein Bcl-XS and neuronal apoptosis induced by serum deprivation are blocked by inhibition and/or down-regulation of NF-kappaB and by prevention of K+ loss. Thus, a direct action of K+ on NF-kappaB/DNA binding regulates gene transcription related to neuronal apoptosis.