Histologic and autoradiographic studies were performed to investigate the cellular reactions during wound healing of the brain of rats after cryonecrosis. A parietal lobe was frozen for 30 s with a cryoprobe at a temperature of -196 degrees C. The survival time ranged between 12 h and 21 days. One h before killing, tritiated thymidine was injected intraperitoneally. Stripping film autoradiograms of the brain sections were made in the usual manner. A typical cryonecrosis develops 12 h after local freezing surrounded by an edematous tissue layer with activated mesenchymal and neurologlial cells. After 10 days a pseudocyst develops surrounded by highly cellular glial tissue. The pseudocyst is still recognizable 3 weeks later but without any remarkable cellular reaction. Autoradiographically the labeling indices of the fibroblasts, leptomeningeal,and endothelial cells in the periphery of the necrosis and the labeling indices of neuroglial, perivascular connective tissue, and microglial cells in the perinecrotic zone increase after 12 h and have their maximum between the 2nd and 3rd postoperative day. In the nonfrozen contralateral cerebral hemispheres, the labeling indices of neuroglial and mesenchymal cells show small peaks in the first 3 postoperative days,also. This can be explained by accompanying edema. There are no remarkable differences between frozen and nonfrozen parts of the brain 3 weeks after local freezing. The results underline the rapid repair of cryonecrosis.