The effect of cortical lesions on the c-fos mRNA expression in adult rat brain was studied using in situ hybridization and Northern blot analysis. It was observed that the single cortical lesion evoked a rapid increase in the content of c-fos mRNA in whole brain. However, when rats received a second and third cortical injury 4 and 8 h after the first one, there was reduced elevation of c-fos mRNA after the second (60%) and third (75%) injuries in comparison with those seen after a single cortical lesion. These results represent the desensitization period during which the induction of c-fos mRNA by repeated cortical lesions was decreased. On the contrary, when animals were treated with a fourth cortical lesion 24 h after the first injury, with recovery for 16 h after the third one, the levels of c-fos mRNA were again elevated to a degree almost comparable to that seen after the single injury alone. The anatomical distribution of c-fos mRNA-expressing cells after cortical lesions revealed a large increase of hybridization to a heterogeneous population of hippocampal neurons including small cells of stratum granulosum in the dentate gyrus and larger cells within hippocampal stratum pyramidale. A dramatic labeling of neurons with c-fos riboprobe in frontoparietal and piriform cortex was also observed but with predominantly localized hybridization to c-fos mRNA on the damaged side of the cortex following the first and second cortical injury. However, hippocampal distribution of c-fos mRNA-containing cells was bilateral as a result of polysynaptic potentials evoked by cortical lesions.