Objective To track the effects of hyperbaric oxygen (HBO) therapy on neonatal rats with hypoxic-ischernic brain damage (HIBD).Methods A total of 126 healthy,seven-day-old SD rats were used to model the HIBD and then randomly divided into seven groups of 18:a sham group,a hypoxic-ischemic (HI) group,a 6 h HBO group,a 24 h HBO group,a 48 h HBO group,a 72 h HBO group and a 1 week HBO group.One hour of HBO treatment at 2 atmospheres absolute pressure was administered once daily for a week to the rats in the latter 5groups,starting at 6,24,48,72 hours and 1 week post the HIBD modeling,while no HBO was administered to the sham and HI groups.The effects were assessed in terms of histological changes ( the neuron density and the percentage of neuron apoptosis in the CA1 region of the hippocampus) and nitric oxide (NO),malondialdehyde (MDA) and superoxygen dismustase (SOD) concentrations after 15 days.Results In the 6 h HBO,24 h HBO,48 h HBO and 72 h HBO groups,average neuron density in the CA1 region was significantly higher than in the HI group.But in the 1 week HBO group the average density was not significantly different than in the HI group.In the 6 h HBO,24 h HBO,48 h HBO and 72 h HBO groups the average percentage of neuron cell apoptosis in the CA1 area of the hippocampus was significantly lower than in the HI group,but the 1 week HBO group again showed no significant difference.There were significant differences in average NO,MDA or SOD levels among the 6 h HBO,24 h HBO,48 h HBO,72 h HBO and HI groups,but the 1 week HBO group showed no significantly higher average levels than the HI group.Conclusions The optimal therapeutic window for using HBO to protect against HIBD is within the first week.The best effect can be obtained in the first 6 hours,but after 1 week HBO no longer has a significant effect.The earlier the HBO is administered,the better the effect obtained.