Previous studies showed that hyperbaric oxygen (HBO) promoted cell proliferation in hypoxic-ischemic (HI) neonate rats. Neural stem cells (NSC) existed in the brain lifelong and can be activated. This study was undertaken to assess whether HBO treatment promoted the proliferation of NSC and repaired the brain damage regardless of when it is started, thus to explore the therapeutic window of HBO treatment. Seven-day-old Sprague-Dawley rats underwent left carotid ligation followed by 2 h of hypoxic stress (8% O(2) at 37 degrees C). Hyperbaric oxygen therapy was administered 3, 6, 12, 24, and 72 h after HI. 5-bromo-2'-deoxyurindine and 5-bromo-2'-deoxyuridine/nestin were detected by immunofluorescence and nestin was examined by western blot analysis 10 days after HI. T-maze forced alternation, the foot-fault test, and the radial arm maze were conducted at P 22 days (14 days after HI), P 30 days, and P 34 days. Thereafter, cerebral morphology was examined by Nissl-staining 28 days after HI. There were remarkable increases in the proliferation of neural stem cells in the HBO-treated group, 3, 6, 12, and 24 h after HI, as compared with the HIBD group. The HBO-treated group, 3, 6, and 12 h after HI, performed better in the behavioral test and had less neural loss in the hippocampal CA1 region as compared with the HIBD group. The therapeutic window for effective HBO treatment could be delayed up to 12 h after HIBD, while the effect decreased 24 h after HI.