GABAergic medium-sized striatal output neurons (SONs) provide the principal output for the neostriatum. In vitro and in vivo data indicate that spike discharge of SONs is tightly controlled by effective synaptic inhibition. Although phasic GABAergic transmission critically depends on ambient GABA levels, the role of GABA transporters (GATs) in neostriatal GABAergic synaptic transmission is largely unknown. In the present study we aimed at elucidating the role of GAT-1 in the developing mouse neostriatum (postnatal day (P) 7–34). We recorded GABAergic postsynaptic currents (PSCs) using the whole-cell patch-clamp technique. Based on the effects of NO-711, a specific GAT-1 blocker, we demonstrate that GAT-1 is operative at this age and influences GABAergic synaptic transmission by presynaptic and postsynaptic mechanisms. Presynaptic GABABR-mediated suppression of GABA release was found to be functional at all ages tested; however, there was no evidence for persistent GABABR activity under control conditions, unless GAT-1 was blocked (P12–34). In addition, whereas no tonic GABAAR-mediated conductances were detected in SONs until P14, application of a specific GABAAR antagonist caused distinct tonic outward currents later in development (P19–34). In the presence of NO-711, tonic GABAAR-mediated currents were also observed at P7–14 and were dramatically increased at more mature stages. Furthermore, GAT-1 block reduced the median amplitude of GABAergic miniature PSCs indicating a decrease in quantal size. We conclude that in the murine neostriatum GAT-1 operates in a net uptake mode. It prevents the persistent activation of presynaptic GABABRs (P12–34) and prevents (P7–14) or reduces (P19–34) tonic postsynaptic GABAAR activity.