Abstract： Objective To investigate changes in the expression of Fas and caspase-3 in the brain of rats with acute CO poisoning and also to provide new evidence for pathogenic mechanism of delayed neuropathological sequelae (DNS) after CO exposure.Methods Fifty SD male rats were randomly assigned to the control group, the 1, 3, and 7 d CO exposure groups and the 7 d HBO treatment group, each consisting of 10 rats. HE, immunohistochemical staining and Western blot were performed to study the expression of Fas and caspase-3 in the brain tissue, following intoxication at various time points.Results The expression of both Fas and caspase-3 were very low in the control group. One day after CO exposure, the expression of both Fas and caspase-3 increased quickly and reached peak 3 days after exposure. No significant differences in both Fas and caspase-3 expressions could be noted between the 3 d and the 7 d exposure groups. For the HBO treatment group, the expression of both Fas and caspase-3 decreased more significantly (P<0.01) as compared with that of the 7 d exposure group, but was still higher than that of the control group(P<0.05). Changes in the expression of Fas and caspase-3 were closely correlated with the severity of brain injury.Conclusions The expression of Fas and apoptosis might be involved in the mechanism of DNS following CO poisoning. HBO seemed to play an important role in the alleviation of brain damage via decreasing the expression of Fas and inhibiting the apoptosis of cells.