Following a mechanical lesion of the left cerebral hemisphere, newborn male rats received a single injection of recombinant rat interferon gamma (IFN gamma) into the lesion cavity at doses of 5, 50 and 500 U. One or two days after the injury the rats were injected with 3H-thymidine. Brain sections were immunostained for glial fibrillary acidic protein (GFAP), subjected to autoradiography and examined microscopically to record proliferating GFAP-immunopositive (GFAP+) astrocytes labeled with 3H-thymidine. Following the intermediate 50 U dose of IFN gamma, numbers of GFAP+ astrocytes and of their mitoses on day 1 after injury were significantly higher than in controls. Nevertheless, the astrocyte labeling index remained at the control level. Injections of the minimal 5 U or the maximal 500 U doses of IFN gamma had no effect on that day. On day 2, however, each of the three doses evoked a statistically significant but dose-independent reduction of the labeling index without similar changes in total number of GFAP+ astrocytes or in numbers of their divisions. The changes appear to indicate a non-linear relation between the intensity of reactive behaviour of astrocytes and the amount of IFN gamma injected into the lesion area. On the basis of previous publications, IFN gamma effects on the astrocyte reactivity are discussed as being age-dependent.