The cerebellar cortices of young weaver mice were examined by electron microscopy and compared with those of normal littermates. Unattached dendritic spines arising from Purkinje cells, very much like those previously described in the adult weaver, were found by the eleventh day after birth. Granule cell degeneration was seen but not examples of degeneration of presynaptic structures could be detected. Unlike either the adult weaver or the developing normal mouse, symmetrical junctions between dendritic spines were present, as were asymmetrical junctions between Purkinje cell dendritic spines and nonspecialized regions of the surface of external granule cell soma. The mechanism of formation of the unattached spines is discussed.