In the adult mammalian CNS, GABA is the main inhibitory transmitter. It inhibits neuronal firing by increasing a Cl- conductance. Bicuculline blocks this effect and induces interictal discharges. A different picture is present in neonatal hippocampal neurones, where synaptically released or exogenously applied GABA depolarizes and excites neuronal membranes--an effect that is due to a different Cl- gradient. In fact, during the early neonatal period, GABA acting on GABAA receptors provides most of the excitatory drive, whereas excitatory glutamatergic synapses are quiescent. It is suggested that during development GABA exerts mainly a trophic action through membrane depolarization and a rise in intracellular Ca2+.