Regional blood flow (F) was measured in avian sarcoma virus (ASV)-induced brain tumors in rats. Blood flow was variable in individual as well as different tumors; tumor F did not correlate with histologic classification, tumor size, central versus peripheral tumor regions, intraparenchymal location, cell density, or specific cytologic characteristics. Low values of F did correlate with tumor necrosis and hydrocephalus; high values of F correlated with tumorous invasion or association with choroid plexus. Mean tumor F was not significantly different from that of the same anatomic, tumor-free brain region of the contralateral hemisphere, but F in brain tissue adjacent to the tumor was depressed significantly. Depression of F was observed in tumor-free cortex and corpus callosum, especially in the hemisphere ipsilateral to the main tumor mass and in those animals with hydrocephalus.