Recent data provide a role for KCC2 in dendritic spine formation and, thereby, in excitatory synaptogenesis. Here we investigated whether this cation-chloride cotransporter is involved in inhibitory synapse formation. To visualize inhibitory synapses, we co-electroporated a molecular construct coding for gephyrin, a major component of the postsynaptic protein network in inhibitory synapses, with a plasmid coding for KCC2 into progenitors of layer 2/3 pyramidal neurons by means of in utero electroporation in rats. To reveal detailed neuronal arbor architecture, electroporated neurons were iontophoretically injected using Lucifer Yellow. Confocal microscopy was used to analyze spatial distribution and density of gephyrin clusters along with their relation to dendritic spines. We found out that precocious expression of KCC2 leads to decreased gephyrin cluster densities in pyramidal neurons associated with an increased dendritic spine density. These observations suggest a role for KCC2 in the establishment of excitation/inhibition balance during neural circuitry development.