Arteriovenous malformation (AVM) may change the cerebral hemodynamics. The purpose of this study was to detect the effects of ionizing radiation (IR) on tissues surrounding AVM in a rat caroticojugular fistula model. Forty rats were divided into four groups. Eight weeks after caroticojugular fistulas and chronic hypoperfusion were created in groups 1 and 2, IR was administered to groups 1 and 3. Group 4 was the control. Brain tissue samples were taken 72 h after irradiation. Comet assay to detect DNA strand breaks (DSB), terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling ( TUNEL) assay for apoptosis, and free radical measurement were performed. Although the difference between fistula plus irradiation ( group 1) and fistula ( group 2) was statistically insignificant in terms of DSB and free radical measurement, apoptotic cell count was significantly higher in group 1. Nonetheless, apoptotic cell count corresponded well with both free radicals and DSB in the irradiated group ( group 3). Ionizing radiation resulted in significant apoptosis in both groups with or without fistulas. Chronic hypoperfusion might not prevent cerebral damage after IR. Optimal care should be taken with brain tissue around AVM during radiotherapy, regardless of presence or absence of the "steal" phenomenon.