The ability of synaptically released GABA to facilitate action potential generation in striatal projection neurons was studied in brain slices using current-clamp, gramicidin-perforated whole cell recordings. Evoked GABAergic postsynaptic potentials (PSPs) were pharmacologically isolated with ionotropic glutamate receptor antagonists. Subthreshold depolarizing current injections were paired with GABAergic PSPs at different intervals. GABAergic PSPs were able to convert current injection-induced depolarizations from subthreshold to suprathreshold, but only when they preceded the current injection by an appropriate interval; accordingly, action potentials were observed 4-140 ms after the onset of the GABAergic PSP, and their likelihood was maximal after 50-60 ms. The GABAergic excitatory effects were fully blocked by the GABA(A) receptor antagonist bicuculline. Appropriately timed GABA PSPs decreased the time taken by current injections to depolarize projection neurons, causing an apparent reduction in the spike threshold. In control solution, the ability of evoked PSPs (comprising both glutamatergic and GABAergic components) to reach spike threshold was often impaired by bicuculline. We conclude that GABAergic PSPs can exert excitatory effects on projection neurons and that this ability crucially depends on the timing between the GABAergic event and a concomitant depolarizing input.