1. Modafinil, a putative central alpha 1 adrenergic agonist, was tested in idiopathic hypersomnia and narcolepsy. 2. Sleep attacks and drowsiness were significantly decreased in 83% of 18 hypersomniac subjects and 71% of 24 narcoleptics. 3. When cataplectic episodes were not totally suppressed the association of a low dose of Clomipramine was successful in improving them. 4. Modafinil, used for at least 3 years in some patients, produces, in most cases, no peripheric sides effects, does not disturb night sleep and is never responsible of tolerance of drug dependence.