Sedative effect of the aqueous extract of Centranthus longiflorus ssp. longiflorus (Cle-1) on intact and adrenalectomized rats was investigated using a thiopental sleeping test to clarify the relationship of this effect on adrenal gland hormones, particularly glucocorticoids. Adrenal gland hormones were found to play an important role in inhibiting the sedative effect of the investigated drugs. It is clear, however, that these hormones are not glucocorticoids. Glucocorticoids were not responsible for shortening the sleep period.