Methionine peptidomimetic derivative (I), is new. Use of a peptidomimetic derived from methionine of formula (CH 3-S-CH 2-CH 2-CH(R)-C(=O)-NH-CH(R1)-CH 2-CH 2-S-CH 3) (III) is claimed as a cosmetic agent to protect and/or stimulate mitochondria of skin cells and in dermatological composition for treatment of cutaneous signs of aging, R : (X-C(O)-NH-) X : alkyl or 1-8C alkoxy and R1 : H or -C(O)R2 R2 : Oalkyl(1-4C) or NH 2. An independent claim is also included for new compounds (III: R1 = H X = alkyl or 1-4C alkoxy). ACTIVITY : Dermatological. MECHANISM OF ACTION : None given.