Multisensory interactions have recently been demonstrated in putatively unisensory neocortical regions including the primary cortices. While the aquisition of multisensory functions and maturation of cross-modal processing in high order convergence regions (e.g. superior colliculus) critically depend on minimal multisensory experience during defined periods, the developmental mechanisms of cross-modal integration in primary sensory cortices are still unknown. We focused on the convergence of visual and tactile information in the primary somatosensory (S1) and visual (V1) cortices of Brown Norway rat. Light flashes and whisker deflections were combined with multi-site extracellular recordings from S1 and V1 in vivo. First, we assessed whether minimal cross-modal exploration is mandatory for the maturation of multisensory functions by focussing on the pre-juvenile developmental stage (retina light sensitive, eyes closed, no whisking). Similar to previous data from adult rats, cross-modal processing in the S1 of pre-juvenile rats relies on modulation of network oscillations via cortico-cortical projections. Second, we elucidated the role of unisensory experience for cross-modal processing by cutting the whiskers of neonatal rats and thus, depriving them of tactile inputs during the critical period. The sensory deprivation significantly changed the cross-modal processing in the S1. Consequently, we propose unisensory development rather than active cross-modal exploration is mandatory for the correct development of multisensory processing in primary cortical areas.