Reeler, an autosomal recessive mutation in mice, is characterized by abnormal positioning of the neurons in the cerebral cortex. We performed a descriptive analysis on the arborization of dendritic processes of large pyramidal neurons in the motor cortex (hindlimb area) of normal and reeler mice, as seen in the Golgi preparations. In the normal mouse, somata of large pyramidal neurons were located in the layer V, and their apical dendrites ascend vertically to the pial surfaces. Their basal dendrites proceed horizontally or inferiorly. In the reeler mouse, typical large pyramidal neurons with a normal (upright) apical dendrite and a variety of atypical large pyramidal neurons with a disoriented apical dendrite were radially scattered within the motor cortex. Typical large pyramidal neurons occupied the lower half of the motor cortex, whereas atypical large pyramidal neurons were predominantly observed in the upper half of the motor cortex. Atypical large pyramidal neurons were further divided into inverted, tumbled, V-shaped, bipolar and superficial polymorphic cells, as previously reported (Terashima et al., J. Comp. Neurol.218: 314-326,1983). Superficial polymorphic cells localized in the layer of polymorphic cells and the layer of the large pyramidal cells were characterized by the extremely poor dendritic arborizations and the smooth surface of the dendrites, which suggests development of dendrites of these neurons was deranged by the reeler genetic locus.