The dentate gyrus is the main hippocampal input structure receiving strong excitatory cortical afferents via the perforant path. Therefore, inhibition at this 'hippocampal gate' is important, particularly during postnatal development, when the hippocampal network is prone to seizures. The present study describes the development of tonic GABAergic inhibition in mouse dentate gyrus. A prominent tonic GABAergic component was already present at early postnatal stages (postnatal day 3), in contrast to the slowly developing phasic postsynaptic GABAergic currents. Tonic currents were mediated by GABA(A) receptors containing α(5)- and δ-subunits, which are sensitive to low ambient GABA concentrations. The extracellular GABA level was determined by synaptic GABA release and GABA uptake via the GABA transporter 1. The contribution of these main regulatory components was surprisingly stable during postnatal granule cell maturation. Throughout postnatal development, tonic GABAergic signals were inhibitory. They increased the action potential threshold of granule cells and reduced network excitability, starting as early as postnatal day 3. Thus, tonic inhibition is already functional at early developmental stages and plays a key role in regulating the excitation/inhibition balance of both the adult and the maturing dentate gyrus.