Memories are believed to be formed in neuronal networks by synaptic modification such as functional changes in synaptic transmission and/or structural changes in synaptic connection.For example, long-term depressionat the parallel fiber-Purkinje cell (PF-PC)synapse has been reported to contributeto the cerebellar motor memory formation.However, few in vivo evidences relate the synaptic or structural changes of the PF-PC synapses to the memory formation.In orderto study these correlated changes,we employed learning model of Horizontal Optokinetic Response (HOKR) that shows adaptationwhich is localized to cerebellerflocculus (FL).Motor learning was induced using modified training protocols to form various amount of long-term memory.Adult male mice were subjected to 4 sessions of 15 min HOKR training with varying rest intervals (0, 10, 20, 40 and 60 min) that constitutes MASSED and SPACED trainings, respectively.Although immediate HOKR adaptation on dayl was identical among all training paradigms, the amount of HOKR adaptation persisted on day2 showed strong positive correlation withthe resting intervals during training.Electron microscopic analysis of PF-PC synapseson day2 revealed a strong negative correlation between retention of HOKR and the reduction in synapse density which was 51%,40% and 20% for trainings with 60,40, and 0 min intervals,respectively.This indicatesthat robust and quickly established structural changes of these synapses lead to synapse elimination underlying long-term adaptation of HOKR.