Prenatal X-irradiation can induce severe microcephaly in the brains of offspring. The possible alteration of neuronal synapse formation was examined in such X-irradiated rats with microcephaly using the whole-cell current clamp technique. The total number of neocortical cells from prenatally (E16) X-irradiated rats decreased to 16% of the control value, while the ratio of GABA-positive/MAP2-positive neurons increased 2.2-fold. Neocortical neurons from E17 normal rat fetuses cultured on monolayers of astrocytes for 7-10 days exhibited synchronized synaptically-driven rhythmic depolarizing potentials (RDPs). Neocortical neurons from prenatally (E15 or E16) X-irradiated rats also exhibited synchronized RDPs, however, their amplitude and the number of spikes decreased. These results suggest that, although neurons which survive in X-irradiated rats can form synapses, inhibitory inputs are predominant over excitatory inputs. It is possible that not only acute neuronal loss induced by X-irradiation but also increased inhibitory inputs in neocortex give rise to subsequent neurological disorders in X-irradiated rats.