Abstract： Objective To investigate the effect of hyperbaric oxygen pretreatment on the latency of acute oxygen toxicity in rats and explore its mechanism. Methods Adult male SD rats were randomly divided into 2 groups: the control group and the hyperbaric oxygen pretreatment (HBOP) group. Rats in the HBOP group were exposed to pure hyperbaric oxygen for 20 min and then were decompressed to normal pressure and ventilated with air for another 20 min for a total of 4 sessions. Then, after 24 hours, the two animal groups were exposed to hyperbaric oxygen at a pressure of 0.5 MPa for 40 min. Measurement indices included changes in the latency of oxygen convulsion, activity of superoxide dismutase(SOD) and also contents of nitric oxide(NO)in the rat brain. The levels of NO and SOD in the brain tissue were also measured. HE staining was used to detect histological changes in the brain tissue. Results HBO pretreatment could significantly prolong the latency of oxygen convulsion, and also markedly reduce the content of nitric oxide(NO). SOD activity and HE staining indicated that no statistical differences could be seen in the 2 groups. Conclusions HBO pretreatment could prolong the latency of oxygen convulsion, the mechanism of which might suggest that HBO exposure could decrease the content of NO in the brain tissue.