Modafinil is a selective wakefulness-promoting agent with beneficial effects in narcolepsy and conditions of sleep deprivation. In a double-blind study we examined its effects in 30 healthy, non sleep-deprived students (19 men and 11 women, aged 19-23 years), who were randomly allocated to placebo, 100 or 200 mg modafinil and 3 h later completed 100 mm visual analogue scales relating to mood and bodily symptoms, before and after an extensive battery of cognitive tests (pen and paper and CANTAB). There were no significant differences between the three treatment groups on any of the cognitive tests used in this study. There was a significant post-treatment change in the factor measuring 'somatic anxiety' and in individual ratings of 'shaking', 'palpitations', 'dizziness', 'restlessness', 'muscular tension', 'physical tiredness' and 'irritability', which was mainly due to significantly higher ratings of somatic anxiety in the 100 mg group compared with the other two groups. Further changes in mood were revealed after the stress of cognitive testing, with the 100 mg group showing greater increases in the 'psychological anxiety' and the 'aggressive mood' factors (as measured from the Bond and Lader scales).