The 'Wriggle Mouse Sagami (WMS)' is a new neurological mutant with severe dystonic movements of the trunk and extremities whose pathological characters are transmitted by an autosomal recessive gene (wri). Manifestations first appear at 10 days to 2 weeks after birth and progress until 12 weeks of age. In spite of the severe dystonic movements, no marked abnormalities had been found in the cyto- or myeloarchitecture of the central nervous system or that of the peripheral nerves, except for the impaired development of the dendritic trees of the Purkinje cells. In this study we quantitatively demonstrated decreased synaptic connections of parallel fibers on the dendritic spines of the Purkinje cells as early as 2 weeks after birth. On the other hand, synaptic boutons on the dendritic shafts and somata of the Purkinje cells and synaptic bouton-like structures which contained synaptic vesicles but without synaptic membrane specialization, were significantly increased in the molecular layer at 9 weeks of age. Glutamic acid decarboxylase immunohistochemistry suggested that some of these increased synaptic boutons and other bouton-like structures may have originated in GABA interneurons, such as stellate cells, basket cells and Golgi cells, and in the cerebellar nuclei. Because of the severity of the manifestations, it appears that synaptic alteration in interneurons also occurs in the other parts of the CNS.