Mouse Sarcoma 37 was transplanted to the subdural space of 14 guinea pigs. Evidence of growth was present in all animals sacrificed between the 2nd and 18th day after transplantation. One animal died at the end of the third week, apparently as the result of invasion of the brain by tumor tissue. The pattern of growth in the meninges and brain was similar to that previously observed in the anterior chamber of the eye of guinea pigs, but, after the 9th day the mass of tumor tissue in the brain was greater than that in the eye for comparable periods. There was no evidence of regression of tumor in the brain in contrast with tumor transplanted to the anterior chamber, which usually begins to regress by the end of the 2nd week.