The small G protein Ras, which is a molecular switch in neurotrophic signal transduction, is implicated in synaptic plasticity and synapse development during ontogeny and in the adult nervous system. To characterise the involvement of Ras-dependent signaling in synaptogenesis, the cortical synapse-to-neuron ratio was investigated in synRas mice overexpressing Val12-Ha-Ras in postmitotic neurons (introduced by Heumann, 2000). The number of synapses per neuron was analysed in cortical layers II/III of the somatosensory cortex at different stages of postnatal development by stereological methods. The synapse-to-neuron ratio was still identical in wild-type and synRas mice at postnatal day 4 before the onset of transgene expression. At P12, P47 and in the adult, analyses revealed a significant increase in the synapse-to-neuron ratio in synRas mice which correlated with the strength of transgene expression. The data presented here provide evidence that Ras activity might be profoundly involved in synaptogenesis by reinforcing the formation or maintenance of synapses during the development and in the adult.