Habituation is a simple form of procedural learning that has been intensively studied in invertebrates,like Aplysia.The cellular and circuit mechanisms underlying habituation in mammals remained largely unknown.Here we explored the habituation of the acoustic startle response in mice.Using two-photon imaging-based in vivo electrophysiological recording,we find that repeated sound stimuli induce both behavioural habituation of the acoustic startle response and reduction of sound-evoked excitatory postsynaptic potentials(EPSPs)in Purkinje neurons of the cerebellar vermis.The behavioural habituation and the reduction of EPSPs show very similar time courses.At the cellular level,blocking GABAa receptor-mediated synaptic inhibition by pharmacological treatment abolishes the reduction of EPSPs in Purkinje neurons.At the behaviour level,mice with specific deletion of GABAa receptors in Purkinje neurons do not exhibit habituation.These results suggest an essential role of synaptic inhibitory potentiation for habituation of the acoustic startle response,thereby endowing physiological relevance to this long-established cellular plasticity(Kano et al.,1992).Finally,optogenetically-induced inhibition of Purkinje neurons clearly causes the reduction of the acoustic startle response.Thus,we identify an inhibitory microcircuit between Purkinje neurons and local interneurons of a cerebellar subregion,the vermis,as the distinct site of habituation.