Neuronal activity is critical for many aspects of brain development. It has often been assumed that the primary excitatory transmitter driving this activity is glutamate. In contrast, we report that during early development, synaptic release of GABA, the primary inhibitory neurotransmitter in the mature brain, is not only excitatory but in addition plays a more robust role than glutamate in generating spike activity in mouse hypothalamic neurons. Based on gramicidin perforated whole cell and extracellular recording, which leave intracellular Cl(-) unperturbed in brain slices and cultures, the GABA(A) receptor antagonist bicuculline induced a dramatic decrease in spike frequency (83% decrease) in developing neurons, three times greater than that generated by glutamate receptor antagonists 2-amino-5-phosphono-pentanoic acid and 6-cyano-7-nitroquinoxalene-2,3-dione. Thus a number of factors related to spike-dependent stabilization of neuronal connections, including Hebbian mechanisms, that are generally applied to glutamate transmission may also participate in stabilization of GABA circuits.