The aim of this research was to investigate the effects of modafinil,a new wake-promoting agent,on vestibular function during 24 h sleep deprivation(SD)so as to provide experimental evidence for the rational use of this drug among air crew.Eight young,healthy male volunteers were exposed to two 24 h periods of continuous wakefulness during the crossover experiment.Initially,200 mg dose of modafinil was given.and one week later,a matching placebo was administered.The SD time started from 08:00 of the first day to 08:00 of the second day.Drugs were given at 0:00 on the second day.Vestibular function was tested at 21:00 on the first day and 1,3,5,7 h after drug administration.The accuracy of saccade tracking and gains in visual-vestibular optokinetic reflex(VVOR)and optokinetic nystagmus(OKN)in the placebo group decreased during 24 h SD,especially at 01:00-05:00 on the second day,while OKN gains in the modafinil group increased significantly.There were no significant difierences in the other vestibular functional indices between the modafinil group and placebo group.The 24 h SD can influence vestibular function to a certain degree,but modafinil may improve OKN.