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Objective:To discuss the DNA-strand breaks at early stage of middle cerebral artery occlusion/reperfusion (MCAO/R). Methods: Neurons number and morphologic change were observed by Nissl stain method, and DNA strand breaks were in situ detected by using DNA polymerase- I Klenow fragment-mediated nick end-labelling method (Klenow method). Results: Six hours after reperfusion, a few neurons in damaged regions appeared morphologic changes while a few Klenow-positive cells were detected (P＜ 0. 01 ).Twenty-four hours after reperfusion lots of neurons showed morphologic change while the number of Klenow-positive cells immediately and remarkably increased (P＜0. 01). Seventy-two hours after reperfusion the number of neurons decreased significantly and the number of Klenow-positive cells was also less than that peak value, DNA single-strand breaks (SSBs) took place in many Klenow-positive cells, and presumed that DNA SSBs might be an important step in DNA-damage procession which might be induced by free radicals.morphological change, which indicated that lots of neurons had already progressed to irreversible damages when DNA SSBs took place.