The invention concerns the use of at least an oil derived from the seeds of the gourd family selected from the group consisting of Lagenaria, Luffa and Momordica, for preparing a composition designed to inhibit 5.alpha.-reductase activity, and the use of said oil for preparing a composition for treating prostatic hypertrophy, prostatic adenoma, acne, hyperseborrhea, alopecia, and hirsutism. The invention also concerns cosmetic treatment methods, in particular for greasy skin. The invention further concerns the use of at least an oil derived from seeds of the gourd family in a nutritional food composition for humans and/or animals as additive for inhibiting 5.alpha. reductase activity.