AbstractThe postnatal differentiation of the hippocampal formation of theataxicmouse was studied. Brains fromataxicmice (axJ/axJ) and littermate controls (+/?), from 19 to 51 days of age, were either impregnated according to a Golgi‐Cox procedure or sectioned and stained with Weil‐hematoxylin and Darrow Red. The hippocampus and dentate gyrus, although somewhat reduced in cross‐sectional area in theataxicbrain, appeared to have a normal complement of both pyramidal cells and granule cells, respectively. Examination of Golgi‐Cox material showed significant differences in the differentiation of the dendritic tree of both pyramidal and granule cells. At 41 days the height of the apical dendrite of CA1 pyramidal cells in theataxicbrain was 76% of the control value, and the width was only 35%. Similarly, the basal dendritic tree was narrower in theataxicmouse. In the dentate gyrus of the 41‐dayataxicbrain, the height of the granule cell dendritic tree was only 74% of the control value. These and other alterations in the dendritic morphology of both CA1 pyramidal cells and granule cells can be explained by a lack of growth of the dendritic tree during the developmental period studied. These findings are discussed in relation to other studies on intrinsic and extrinsic factors and their effect on normal hippocampal differ