Single oral administration of modafinil (16-64 mg/kg) in monkeys induced an increase in nocturnal activity and in behavioural arousal without stereotyped behaviour. Modafinil-induced increase in nocturnal activity was prevented by the centrally acting alpha 1 adrenoceptor antagonist prazosin. After repeated oral administration of modafinil, b.i.d. for 5 days, nocturnal activity was still increased, but was lower than after a single administration; the gradual decrease appeared on the second night. No rebound of sleep and no residual effect on night 5 (when modafinil was administered for 4 days and placebo on day 5) were seen after the withdrawal of the drug. Therefore, modafinil appeared to produce strong behavioural stimulation and awakening in a non-human primate species, after single, as well as after repeated administration, without other obvious side-effects (no sign of anxiety, no behavioural trouble). These awakening properties involved a modulation (stimulation) of a central alpha 1 adrenergic system.