Single administration of the new drug modafinil was followed by an increase in locomotor activity in mice and in nocturnal activity in monkeys. Stereotyped behaviour in mice and rats, and potentiation of amphetamine-induced stereotyped behaviour were not observed; however, at the highest dose used, a slight potentiation of apomorphine-induced stereotyped behaviour was observed in rats. The modafinil-induced increase in locomotor activity in mice was prevented by the centrally acting alpha 1-adrenoceptor antagonists, prazosin and phenoxybenzamine, and by reserpine but not by the mixed dopamine D-1/D-2 antagonist, haloperidol, the dopamine D-2 antagonist, sulpiride, the peripherally acting alpha 1-adrenoceptor antagonist, phentolamine, the alpha 2-adrenoceptor antagonist, yohimbine, the beta-adrenoceptor antagonist, propranolol, or by the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine. Likewise, the modafinil-induced increase in nocturnal activity in monkeys was prevented by prazosin. Interestingly, modafinil did not produce obvious peripheral sympathetic effects in mice and rats (no salivation, no contraction of the pilomotor muscles, slight mydriasis only at high doses). Therefore, modafinil appears to produce a strong stimulating effect in rodents and in primates. These effects could be linked to modulation (stimulation) of central alpha 1-adrenoceptors unaccompanied by peripheral sympathetic effects, which is unexpected.