Abstract: AIM: To study the effect of dl-3-n-butylphthalide(dl-NBP) on the function of mitochondrial respiratory chain and to elucidate the increasing effect of NBP on brain energy supply during cerebral ischemia. METHODS: Mitochondria were isolated from the brain of transient middle cerebral artery occluded (MCAO) rat and the activities of the four complexes of the respiratory chain were determined. RESULTS: The activity of complex IV was deeply decreased after 1 h-ischemia. It was back to normal level when treated with NBP (5 mg.kg^-1 or 10 mg.kg^-1 ip 10 min before ischemia). During the reperfusion period after ischemia, the activity of complex I was notably increased at 3 h, and that of complex II was decreased at 6 h. With NBP treatment, these altered activities also returned to normal level. In cultured neurons subjected to 6 h-hypoxia/hypoglycemia, the same increasing effect of NBP(d-,l- or dl-) on the activity of complex IV was also found, and d-NBP seemed to be more effective. CONCLUSION: NBP can act directly on complex IV to increase its activity. This action may play an important role in the increasing effect of NBP on brain energy supply during cerebral ischemia.