It is speculated that GABA-dependent excitation is important for brain development. The action of GABA and glycine critically depends on the Cl- gradients across the plasma membrane and hence on the intracellular Cl- concentration ([Cl-]i) of the particular neuron. The Na-K-2Cl-co-transporter (NKCC) maintains ([Cl-]i) at levels above the predicted electrochemical equilibrium. We tested early GABAergic excitation in hippocampal slices from NKCC1-deficient (NKCC1(/() and wild type (NKCC1+/+) mice at postnatal day (P)1. The shift in the GABA reversal potential of CA1 pyramidal neurons by application of 100 mM GABA was 20.9 ± 2.2 mV in NKCC1+/+ and 5.8 ± 0.7 mV in NKCC1(/( preparations (p<0.02). Ca2+ entry upon GABAergic excitation has been postulated to promote the expression of the K-Cl-co-transporter KCC2. The depolarization-dependent Ca2+ entry normally seen in P1 hippocampal CA1 neurons loaded with Fura-2 was reduced in NKCC1(/( pyramidal cells upon application of GABA (p<0.001), but not glutamate. The expression of KCC2 in NKCC1(/( tissue preparations at embryonic day (E)15, P1, P5, and P15 was unchanged compared to NKCC1+/+. These results demonstrate that NKCC1 is indeed involved in GABAergic excitation during embryonic development but that GABAergic excitation does not promote the expression of KCC2 per se.