GABAergic synapses on Cajal-Retzius neurons in layer I of the murine neocortex experience GABA(B) receptor (GABA(B)R)-mediated tonic inhibition. Extracellular GABA concentration (GABA) that determines the strength of GABA(B)R-mediated inhibition is controlled by GABA transporters (GATs). In this study, we hypothesized that the strength of presynaptic GABA(B)R activation reflects GABA in the vicinity of synaptic contacts. Slices obtained from two age groups were used, namely postnatal days (P)2-3 and P5-7. GABAergic postsynaptic currents (IPSCs) were recorded using the whole-cell patch-clamp technique. Minimal electrical stimulation in layer I was applied to elicit evoked IPSCs (eIPSCs) using a paired-pulse protocol. Three parameters were selected for comparison: the mean eIPSC amplitude, paired-pulse ratio, and failure rate. When GAT-1 and GAT-2/3 were blocked by NO-711 (10 microM) and SNAP-5114 (40 microM), respectively, no tonic GABA(B)R-mediated inhibition was observed. In order to restore the control levels of GABA(B)R-mediated inhibition, 250 and 125 nm exogenous GABA was required at P2-3 and P5-7, respectively. Addition of 3-mercaptopropionic acid, a glutamate decarboxylase inhibitor, did not significantly change the obtained values arguing against the suggestion that a mechanism different from GATs contributes to GABA control. We conclude that juxtasynaptic GABA is higher (about 250 nM) at P2-3 than at P5-7 (about 125 nM). As both radial cell migration and corticogenesis in general are strongly dependent on GABA and the formation of the last layer 2/3 is finished by P4 in rodents, the observed GABA reduction in layer I might reflect this crucial event in the cortical development.