Our previous studies demonstrated that fractured tactile cerebellar maps in rats reorganize following deafferentation during development and in adulthood, while maintaining a fractured somatotopy (Gonzalez et al. 1993; Morissette 1996; Shumway et al. 1999). Several months after deafferentation of the infraorbital branch of the trigeminal nerve, the missing upper lip innervation is replaced in the tactile maps in the granule cell layer of crus IIa. The predominant input into the denervated area is always the upper incisor representation. This study examined whether this reorganization was due to mechanisms intrinsic to the cerebellum or extrinsic, i.e., occurring in somatosensory structures afferent to the cerebellum. We first compared normal and deafferented maps, and found that the expansion of the upper incisor is not due to a preexisting bias in the strength or abundance of upper incisor input in normal animals. We then mapped tactile representations before and immediately after denervation. We found that the pattern of reorganization observed in the cerebellum several months later is not due to unmasking of a silent or weaker upper incisor representation. Both results indicate that the reorganization is not a result of subsequent growth or sprouting mechanism within the cerebellum itself. Finally, we compared postlesion maps in the cerebellum and the somatosensory cortex. We found that the upper incisor representation significantly expands in both regions, and that this expansion is correlated, suggesting that reorganization in the cerebellum is a passive consequence of reorganization in afferent cerebellar pathways. This result has important developmental and functional implications.