The aim of this study was to evaluate how timing of irradiation after brain surgery in rats relates to overall extent of brain radiation damage. Extent of injury was determined according to lipid peroxidation ( malondialdehyde; MDA) levels in brain tissue. Thirty female rats were randomly assigned to five equal groups ( Groups A - E). Four groups underwent trephination and scalpel hemisection of right frontal lobe. Rats in Groups B and D received 25 Gy cranial irradiation in a LINAC system at 10 and 20 days after surgery, respectively. Twenty-four hours later they were killed and their right frontal lobes were removed for lipid peroxidation determination. Groups A and C were not irradiated; these groups were killed and had their frontal lobes removed on day 11 and day 21 post-surgery, respectively. The remaining six animals ( Group E, sham surgery) underwent trephination only, and were killed and had their frontal lobes removed 24 h later.