Abstract: A promising strategy for combating pathologies associated with oxidative stress is the use of mitochondria-targeted drugs. One of them is the positively charged tetrapeptide SS-20, which is able to penetrate into mitochondria. The effect of SS-20 on isolated supercoupled rat-liver mitochondria is studied. It is shown that SS-20 decreases hydrogen peroxide production, inhibits the opening of the nonspecific Ca2+/Pi-dependent mitochondrial pore, does not exhibit uncoupling activity, does not inhibit mitochondrial respiration, and does not reduce their membrane potential. In the aerobic yeast Dipodascus magnusii, SS-20 reduced the level of oxidative stress in yeast and prevented prooxidant-induced mitochondrial fragmentation. © 2025 Elsevier B.V., All rights reserved.