The ultrastructural and biochemical changes in the brain tissue of 20 mice were studied. The mice, separated into acute and chronic groups, were injected with phorbol myristate acetate (PMA) to trigger the release of free radicals. Malondialdehyde measurement and electron microscopic examination were applied for the evaluation of the effects of the free radicals. The level of lipid peroxide in the chronic PMA group was found to be significantly higher than it was in the acute PMA group (P<0.005). An electron microscopic examination of the acute group revealed disruption of the mitochondrial cristae and dilatation of the endoplasmic reticulum in the neurons. Myelin sheaths of the nerve fibers exhibited focal structural changes. Neurons and neuroglial cells in the chronic group, however, exhibited distinct ultrastructural alterations. The ultrastructural and biochemical findings showed that free radicals lead to brain damage.