Mossy fibre synapses on the CA3 hippocampal neurons in the brain of ground squirrels repeatedly undergo a striking structural transformation during hibernation. In the middle of hibernation bout the giant complex mossy fibre synapses have a reduced number of dendritic spine infoldings that are smaller and have a decreased number of postsynaptic densities in comparison with mossy fibre synapses of active animals. Two hours after arousal all these parameters of mossy fibre synapses increase and significantly exceed their levels not only in torpid but in active euthermic animals between bouts of torpor. The longer postsynaptic densities and the greater proportion of perforated postsynaptic densities were found soon after arousal. These rapid, reversible and repeated changes indicate a cyclic process of partial denervation/reinnervation of hippocampal neurons by mossy fibres in the course of the innate, stereotyped behaviour.