Changes in the number and distribution of microglial cells proliferating in response to unilateral injury of the cerebral hemisphere were investigated in 30-day-old rats. Twelve hours or 1, 2, 4, or 8 days following the injury the rats were injected with H-3-thymidine and killed 4 hr later. Brain sections were processed for BSI-B4 isolectin histochemistry followed by autoradiography, During microscopic observations, four morphological types of lectin-positive and autoradiographically labeled cells were distinguished: 1) ramified, 2) hypertrophic, 3) bushy, and 4) macrophages, Subsequently, numbers and locations of the cell types within the injury area were recorded at different stages of the experiment. The earliest signs of the proliferative response were displayed 12 hr after injury by ramified microglia, On the first day after injury, those cells represented about 50% of the whole cell population and were spread at relatively the longest distances from the lesion site, During subsequent stages of the response, a considerable reduction of the area occupied by proliferating microglia corresponded with a dramatic quantitative decrease of their ramified fraction and a simultaneous increase of their more advanced reactive forms and macrophages.