A major event in the maturation of CNS GABAergic transmission is the qualitative change in GABA(A)-mediated responses from depolarizing to hyperpolarizing. In cortical regions, this is attributed to the increased expression of potassium chloride cotransporter 2b (KCC2b), the main isoform of the neuron-specific K-Cl cotransporter KCC2. We have previously shown that transcription factor early growth response 4 (Egr4) can activate the KCC2b promoter. Here we demonstrate that in immature hippocampal neurons BDNF robustly induces ERK1/2 (extracellular signal-regulated kinase 1/2)-dependent Egr4 expression and rapid Egr4-dependent activation of the KCC2b promoter. The subsequent increase in KCC2b mRNA contributes to the expression of total KCC2 protein levels. These results indicate that Egr4 is an important component in the mechanism of BDNF-dependent KCC2 gene regulation via the ERK1/2 pathway in immature neurons.