REPAIR of mechanically injured primary somatosensory cortex in 3 week old mice was studied by placing small, solid foetal neurotransplants into large cortical cavities. After transplantation, the graft and host tissues were distinguished immunocytochemically owing to their expression of two different Thy-1 antigens. Cell proliferation was monitored by 3H-thymidine autoradiography. The following observations were made two months after operation: (i) In 8 out of 11 grafted animals new cortical tissue had taken the place of the cavity. (ii) Five of these 8 animals contained only host tissue; the remainder presented a small piece of grafted tissue. (iii) In the restored cortical area, newly generated cells were predominantly of host origin. These data suggest that the restorative capacity of the already post-mitotic cerebral cortex is not lost and may be reactivated. The presence of a foetal neural graft seems to favour this process.