The effects of hexasulfobutylated C₆₀ (FC₄S), a free radical remover, on the total volume infarct size elicited by the damaging effects of focal cerebral ischemia were studied on Long-Evans rats in vivo. FC₄S was administered intravenously either 15 min before middle cerebral artery (MCA) occlusion (pretreatment groups) or it was injected when the common carotid arteries clips were removed (treatment groups). FC₄S did not alter the pH, blood gases, heart rate, or mean arterial blood pressure in either pretreatment or treatment groups of the rats. However, after administration of FC₄S at dosages of 10 and 100 μg/kg, the total volume of infarction was significantly reduced in both pretreatment and treatment groups. In addition, after FC₄S administration, the nitric oxide (NO) content in plasma was increased and the lactate dehydrogenase (LDH) levels was decreased. It is concluded that FC₄S may be used as a neuroprotective agent on focal cerebral ischemia. The beneficial effects may be partly related to its antioxidant property and to the upregulation of NO production of the compound.