Recent in vitro studies showed that stem cells might fuse with mature cells or each other; however, there is no in vivo evidence for this phenomenon in the cerebral cortex. Our goal was to find evidence for cell fusion in a model of traumatic brain injury followed by grafting of embryonic cortical cells. Cold lesion protocol was applied to induce lesion of the motor cortex in adult male rats. Six days later we grafted a suspension of freshly isolated rat brain cortical cells of early embryonic stage (E14) into the penumbra area of the lesion. The grafted cell nuclei were labelled with bromodeoxyuridine (BrDU). Six days after transplantation 4,328 BrDU positive cells were observed in nine animals. 89.5% of these cells had cytoplasmic staining probably representing dead or phagocyted grafted cells. Ten percent of surviving BrDU positive cells had only one BrDU positive nucleus and negative cytoplasm, while 0.5% had two distinct nuclei, one was unlabelled and one was BrDU positive. These cells were similar in appearance and size to the astrocytes in the vicinity and expressed the astocyte specific glial fibrillaly acidic protein. Thus, these cells showed a possible sign of cell fusion in the penumbral region of the injured brain.