Abstract: Experiments were conducted on SD rats.Three different limrts of tolerance, point of peak breath failing (group A) ,appearance of first gasping (group B) and last breath (group C) were used as the start of next exposure,respectively.As exposure went on,anmals' state and cortical electrical activity increased in group A while deceased in group B and C.The to lerance time of the second ,third and fourth run was 1.03, 1.03, 1.05 times in group A; 1.20 ,1.50 and 2.16 times in group B;and 1.43, 1.91 and 2.74 times in group C,respectively longer than that of their own first run.These results indicate that adaptation to hypoxia is related to inhibition of the central nervous system and the extent of increase in tolerance seems to be correlated to the extent of inhibition.

Key words: hypoxia tolerance, CSEP, EEG