Epileptogenesis is often a very slow gradual process emerging spontaneously or in response to a given even such as trauma. Most experimental efforts have explored epileptogenesis in response to an initial trauma or seizure event, whereas the mechanisms underlying idiopathic epilepsy remain obscure. It appears likely to involve a very slowly developing shift in the excitation/inhibition balance that reaches a threshold. Here, in order to identify the earliest functional ‘footprint’ of hyperexcitability on the hippocampal circuit, we induced modest hyperexcitation in mouse hippocampal slices using picrotoxin (PTX) and investigated the intrinsic membrane properties as well as the spine structure in CA1 pyramidal neurons after 30 min exposure, before appearance of synchronized discharging events. Modest hyperexcitation did not induce changes in the intrinsic properties of the CA1 pyramidal cells, though we found subtle changes in dendritic spines shapes, that may impact signalling independently of synaptic release and post-synaptic receptor mechanisms. Our further efforts will explore whether these changes may contribute to a slow shift in the excitation/inhibition balance and epileptogenesis, or whether they reflect a beneficial homeostatic response to protect the circuit against hyperexcitability.