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 microg/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.