The object of the present invention is the use of compounds designed toinhibit the activity of the enzyme 5.alpha.-reductase.This is a novel use of compounds of general Formula (I): CH3 (-CH=CH)n-R (I)where n = from 2 to 7, and R is chosenfrom among: CHO, CH2OH, CH2O-CO-R', CO-O(-), where R' is chosen from among H,and the alky from C1 to C22, each compoundof general Formula (I) being used as such or in mixtures with other compounds,as the active principle in a pharmaceuticalor cosmetic composition designed to inhibit the action of the enzyme 5.alpha.-reductase, and of the pharmaceutical or cosmeticcompositions deriving therefrom.