✓ Eleven selected patients with brain tumor were given 3H-thymidine intravenously at the time of operation. Parts of the excised tumor were fixed for routine histological studies as well as for simple radioautography. Other parts were minced immediately after excision and incubated at 37°C with 14C-thymidine, and were thus doubly labeled. Simple radioautography of the tumor specimen showed fairly even distribution of labeled cells in the astrocytoma group and uneven labeling of tumor cells in malignant gliomas, including glioblastomas. In most glioblastomas, the labeling index (percentage of cells labeled with 3H-thymidine) varied from near 0% in the necrotic areas to approximately 20% in the most viable parts of the same tumor; the labeling index of viable parts of the tumor ranged from 5% to 10% with the exception of astrocytomas in which the index was 1% or less. The duration of DNA synthesis ranged from 7 to 10 hours in the majority of tumors regardless of the nature or grade of malignancy.