GABA (gamma-aminobutyric acid) is the main inhibitory transmitter in the adult brain, and it exerts its fast hyperpolarizing effect through activation of anion (predominantly Cl-)-permeant GABA(A) receptors(1), However, during early neuronal development, GABA(A)-receptor-mediated responses are often depolarizing(2,3), which may be a key factor in the control of several Ca2+-dependent developmental phenomena, including neuronal proliferation, migration and targeting(4-6). To date, however, the molecular mechanism underlying this shift in neuronal electrophysiological phenotype is unknown. Here we show that, in pyramidal neurons of the rat hippocampus, the ontogenetic change in GABA(A)-mediated responses from depolarizing to hyperpolarizing is coupled to a developmental induction of the expression of the neuronal Cl--extruding K+/Cl- co-transporter, KCC2 (ref. 7). Antisense oligonucleotide inhibition of KCC2 expression produces a marked positive shift in the reversal potential of GABA, responses in functionally mature hippocampal pyramidal neurons. These data support the conclusion that KCC2 is the main Cl- extruder to promote fast hyperpolarizing postsynaptic inhibition in the brain.