Proliferative response of astrocytes to unilateral injury of the cerebral hemisphere was investigated in 30-day-old rats using a combination of [^{3}H]thymidine autoradiography and immunocytochemical staining for glial fibrillary acidic protein. At different intervals following injury the animals were injected with [^{3}H]thymidine and the distribution of double-labeled cells was recorded 4 h (short-term experiment) or 40-days (long-term experiment) after each injection. Within the region of the lesion a strong reactive proliferation of astrocytes began as early as 2 h after injury, although, at that time all dividing neuroglial cells were located at a relatively long distance from the lesion margin. Forty days later, however, autoradiographically labeled astrocytes were observed in the center of the lesion. This was regarded as evidence for the ability of astrocytes to migrate towards the site of injury. Maximal reactive proliferation of astroglial cells was observed at day 2 after injury and then reached the postmitotic plateau at about day 8. The cells, while dividing, were mostly GFAP-negative. Therefore, the number of astrocytes produced at particular days after injury, and their contribution to post-traumatic scar formation could be assessed only in the long-term experiment, after the newly formed cells became GFAP-positive.