OBJECTIVE The application and the efficacy of hyperbaric oxygen (HBO) in hypoxic-ischemic brain damage (HIBD) remain controversial. This study aimed to explore the effects of HBO on brain functional outcome and possible repair mechanisms in neonatal rats with intrauterine HIBD in aspects of the number of survived neurons and the central nervous electrophysiological conduction velocity. METHODS A rat model of intrauterine HIBD was prepared. Subjects were divided into four groups at random: HIBD, HBO-treated HIBD group, normal control and HBO-treated normal control. After 24 hrs of the operation, the two HBO-treated groups received HBO treatment (0.02 MPa, 1 hr/d) for 14 days. When the rats were 4 weeks old, the electrophysiological changes in the central nervous system (CNS) were observed by brainstem auditory evoked potential (BAEP) for assessing brain function. Hematoxylin and eosin (HE) staining and Nissl's stainting were employed to observe the pathological change and the number of neurons in the hippocampus. RESULTS The peak latency of waves II and IV and the interpeak latency of waves I-IV in the HBO-treated HIBD group were shortened compared with those in the untreated HIBD group (P< 0.05). HE staining displayed that the pathological injuries in the hippocampus were alleviated in the HBO-treated HIBD group when compared with the untreated HIBD group. Nissl,s staining showed that survived neurons in the HBO-treated HIBD group were more than the untreated HIBD group (P< 0.05). The HBO-treated control group showed increased survived neurons compared with the untreated control group (P< 0.05). CONCLUSIONS Early HBO treatment might improve brain functional outcome through increasing synaptic transmission efficiency, improving central nervous electrophysiological conduction velocity and reducing neuron death in neonatal rats with intrauterine HIBD.