Tonic activation of GABA(A) receptors takes place before the development of functional synapses in cortical structures. We studied whether inefficient GABA uptake might explain the presence of a tonic GABA(A)-mediated current (I(GABA-A)) in early postnatal hippocampal pyramidal neurons. The data show, however, that the tonic I(GABA-A) is enhanced by the specific blocker of GABA transporter-1 (GAT-1), NO-711 (1-[2-[[(Diphenylmethyleneimino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride), at birth in rat CA3 pyramidal neurons. NO-711 also prolonged the duration of GABA transients during endogenous hippocampal network events (known as giant depolarizing potentials) at postnatal day 0. The endogenous tonic I(GABA-A) was seen and it was enhanced by NO-711 in the presence of tetrodotoxin, which itself had only a minor effect on the holding current under control conditions. This indicates that the source of interstitial GABA is largely independent of action-potential activity. The tonic I(GABA-A) in neonatal CA3 pyramidal neurons was increased by zolpidem, indicating that at least a proportion of the underlying GABA(A) receptors contain gamma2 and alpha1-alpha3 subunits. The present data point to a significant role for GAT-1 in the control of the excitability of immature hippocampal neurons and networks.