OBJECTIVE: A recent study has suggested that hyperbaric oxygen (HBO) therapy administered within 3 hrs following hypoxic-ischemic brain damage (HIBD) may alleviate brain white matter damage (WMD) in neonatal rats. However it is unclear whether a delayed HBO therapy (more than 3 hrs following HIBD) has neuroprotective effects in neonatal rats. This study aimed to explore the effect of HBO therapy administered at different time points following HIBD on WMD in neonatal rats. METHODS: The HIBD model was prepared according to the Rice-Vannucci procedure in 7-day-old Sprague-Dawley rats. HBO therapy was administered at 3, 6, 12, 24 or 72 hrs after HIBD, once daily for consecutive 7 days. T-maze test, the foot-fault test and the radial arm maze test were performed after 14 days of HIBD. Myelin basic protein (MBP) in the callositas and corpora striata was examined by immunohistochemical method 28 days after HIBD. RESULTS: The rats receiving HBO therapy at 3, 6 and 12 hrs after HIBD performed significantly better in the T-maze test, the radial arm maze test and the foot-fault test than the untreated HIBD rats. There were no significant differences in the behavioral test results between the HBO-treated groups administered HBO at 24 and 72 hrs after HIBD and the untreated HIBD group. The MBP expression in the HBO-treated groups treated within 12 hrs after HIBD was significantly higher than that in the untreated HIBD group (P< 0.05). When the HBO therapeutic window was delayed to 24 hrs after HIBD, there were no significant differences in the MBP expression between the HBO-treated and the untreated HIBD groups. CONCLUSIONS: HBO therapy administered within 12 hrs following HIBD can alleviate brain WMD in neonatal rats, but the efficacy of HBO therapy administered 24 hrs after HIBD does not appear to be satisfactory.［Chin J Contemp Pediatr, 2007, 9 (4):308-312］