Murine sarcoma virus, Moloney strain, was inoculated intracerebrally into 75 Spraque-Dawley rats within 24 hours after birth. Of 48 rats examined histologically within 35 days after virus inoculation, most developed granulomatous lesions in the meninges or in the cortex and in the glial nodules of the subependymal region, or in all these regions. The granulomatous lesions first appeared 8 days after virus inoculation and reached a peak about 30 days after virus inoculation. Thereafter, the lesions healed gradually to scars and disappeared. Of 27 rats that survived beyond 35 days, 10 developed brain tumors at or near the sites of cortical granulomatous lesions and subependymal glial nodules. The interval from virus inoculation to death of tumor-bearing rats was between 40 to 115 days, averaging 84 days. The predilection sites were in the subependymal region and hippocampal cerebral cortex. Three brain tumors were cultured using monolayer and rotation culture techniques. These cultured tumor cells showed morphological features very similar to those of the original tumor cells. THe tumor was classified morphologically into the astrocytoma group by light and electron microscopy and by cytological characteristics of tumor cells in tissue culture.