The small G protein Ras, which is involved critically in neurotrophic signal transduction, has been implicated in neuronal plasticity of both the developing and the adult nervous systems. In the present study, the cumulative effects of constitutive Ras activity from early in postnatal development into the adult upon the morphology of hippocampal pyramidal neurons were investigated in synRas mice overexpressing Val12-Ha-Ras postmitotically under the control of the rat synapsin I promoter. In synRas mice, stereologic investigations revealed hypertrophy of the hippocampus associated with an increase in perikaryal size of pyramidal neurons within the CA2/CA3 region and the gyrus dentatus. Morphometric analyses of Lucifer Yellow-filled CA1 pyramidal neurons, in addition, demonstrated considerable expansion of dendritic arbors. The increase in basal dendritic size was caused primarily by alterations of intermediate and distal segments and was associated with an enlarged dendritic surface. Apical dendrites showed similar but more moderate changes, which were attributed mainly to elongation of terminal segments. Sholl analyses illustrated higher complexity of both basal and apical trees. Despite significant morphologic alterations, dendritic arbors preserve their major design principles. The synaptic density within the stratum radiatum of CA1 remained unchanged; however, increases in the total hippocampal volume and in apical dendritic size imply an increment in the absolute number of synaptic contacts. The data presented here suggest a critical involvement of Ras dependent signaling in morphoregulatory processes during the maturation and in the maintenance of hippocampal pyramidal neurons. © 2004 Wiley-Liss, Inc.