GABA (-aminobutyric acid) is one of the important neurotransmitters in the central nervous system of mammals and its action is variable according to the maturation phases of neurons. The neurons of early cultural days (less than 7 days) have been used for a developing neuronal model, while the neurons of later days (over 3 weeks) used for a mature model. This study was performed to investigate the electrophysiological property of GABAergic synapses in the hippocampal neurons cultured for 10 to 14 days which are considered to be transitional period between the developing and the mature phases. Membrane potential was depolarized and a inward transmembrane current was induced by 20 M GABA infusion. Frequency and amplitude of spontaneous postsynaptic currents (PSCs) were inhibited during the GABA infusion, but decay time constant was not affected significantly. In most hippocampal neurons, no GABAergic PSCs were observed during the administration of 0.5 M TTX, 50 M APV and 10 M CNQX. In the neurons counting 25% approximately, however, small persisted PSCs showed the existence of GABAergic synapses which were blocked by 10 M bicuculine. As the functional property of isolated GABAergic synapses, amplitude of GABAergic PSCs were diminished, and decay time constants and rising times were prolonged during the 20 M GABA infusion in all recorded neurons. In conclusion, approximately 25% of the hippocmpal neurons cultured 10 to 14 days used GABA as well as glutamate as a neurotransmitter. It seems that the GABAergic synapses composed of functionally homogenous GABAA receptors act as inhibitory modulator of the excitatory signal transmission.