The effect of brain trauma on the formation of metastatic growth was examined by iv injection of 6 strains of rat ascites hepatoma. Three of the tumors (AH7974F, AH66F, and AH13) were in the single-cell state and three (AH272, AH7974, and AH601) contained cell aggregates. None of the tumors showed any tumor growth in the intact brain after injection into the tail vein. However, after traumatization of the brain tissue it was possible to observe tumor formation in the lesion, mainly by injection of the single-cell type tumors. The brain tissue was most susceptive to the tumor cells 10 min and 7 days after trauma. On the other hand, after the injection into the carotid artery of traumatized animals, the tumor formation in the brain was found in both tumor groups.