Changes of local cerebral blood flow (l-CBF) were estimated by autoradiographic techniques in middle cerebral artery occlusion rats given a large dose of (3 mg/kg) dexamethasone. The sensory-motor, parietal, auditory and olfactory cortices showed a significant reduction (greater than 25%) of l-CBF. In MCA occlusion with large dose dexamethasone group, ischemic cortices showed significant reduction from the only MCA occlusion group (highest 36%). There was no significant reduction of l-CBF in the deep structures. The effects of dexamethasone on drains with various pathologies have been studied both clinically and experimentally with many kinds of techniques. These effects are now being reevaluated. Still controversial however, is the mechanism that determines whether dexamethasone has beneficial, non-beneficial, or harmful effect. This study may help us to unravel the mechanism of steroid effects, as well as the relationship between corticosteroid and central nervous system.