Small stab wounds were made in the frontal lobe of adult rats by the insertion of a glass capillary or a steel needle with a diameter of 50 mum. Macroscopically, there was a minute lesion filled with red blood cells. Ultrastructurally, there was a prominent edema with 200 mum from the wound during the first 3 days. There was an invasion of neutrophilic leukocytes, monocytes, and pericytes during the first postoperative week and a subsequent phagocytosis of necrotic devris during the first 2 weeks. Neurons within 150-200 mum from the wound degenerated and became phagocytized by macrophages. Astrocytes showed a transitional swelling, later followed by an accumulation of glycogen and filaments. Oligodendrocytes as well as astrocytes took part in phagocytosis. The most prominent early reaction of oligodendrocytes was the formation of multiple vacuoles and, in less injured cells, pinocytotic vesicles. Many cells with a morphology intermediate between the usual appearances of astrocytes and oligodendrocytes were seen. Some of these cells were similar to "third type of neuroglial cells." It was concluded that the healing of this type of small stab wounds occurred in a very regular way. Morpholocially, very little necrosis was seen, but there was a profound reactivity both in invading hematogeneous phagocytes, i.e., neutrophilic leukocytes and monocytes, and in neuroglial cells. Thus, the present type of trauma might be considered a useful model for studying events in healing of brain injuries.