A radioautographic technic is described which has been used in 18 cases to correlate relative uptake of P32 with histologic structure of brain tumors. Surgical or autopsy specimens were obtained 24-72 hrs. after intraven. admn. of 0.5-4 millicuries of labeled inorganic phosphate ion. A modified Linderstrom-Lang method of sectioning was used involving no fixation; fixatives leach out P32 and cause major shifts of the isotope within the tissue. Using data of Steinberg and Solomon, the relative P32 uptake in different areas was quantitated by a system of projection micro-photodensitometry. Uptake of phosphate in normal gray matter always exceeded that in white. Necrotic areas often showed virtually no exchange with p32. Glioblastoma multiforme took up more phosphate than astrocytoma, and tumor tissue averaged 5-100 times the uptake of normal brain. Advantages of this radio-autographic technique as a dynamic method for studying neoplasms are discussed.