The addition of high concentrations of cumene or tert-butyl hydroperoxide to previously deenergized mitochondria results in the energization of these mitochondria and activation of ATP synthesis.The energization effect was observed in the presence of 0.5-0.7 m M cumene hydroperoxide or 2.0-2.5 m M tert-butyl hydroperoxide.This energization of mitochondria and activation of oxidative phosphorylation by organic hydroperoxides required the presence of ADP in the mitochondrial matrix and does not depend upon the method of deenergization of the mitochondria.Organic hydroperoxides are known to be widespread in the animal cells (1, 2).Hydroperoxides have been shown to be formed during lipid peroxidation in biological membranes (3) and in the metabolism of phosphatidylinositol (4-6) and arachidonic acid (7).Multiple pathways for the oxidative transformation of arachidonic acid via (i) cyclooxygenase, (ii) lipoxygenase, and (iii) cytochrome P-450 have been demonstrated.The cyclooxygenase activity of prostaglandin endoperoxide synthase converts arachidonic acid to 15-hydroperoxy-9,11-endoperoxide, and a peroxidase activity of prostaglandin endoperoxide synthase reduces 15-hydroperoxy-9,ll-endoperoxide to its 15-hydroxy analogue (8).The peroxidase activity of prostaglandin endoperoxide synthase catalyzes the reduction of other lipid hydroperoxides (9, 10).Lipoxygenases which convert arachidonic acid to various isomeric hydroperoxyeicosatetraenoic acids (12-HPETE,' 15-HPETE, 5-HPETE) have been identified in a number of cells and tissues (11).In general these intermediates are further converted to leukotrienes (7).However, it has been shown (12) that glutathione peroxidase is directly involved in the reduction of 12-HPETE to 12-hydroxyeicosatetraenoic acid.The participation of cytochrome P-450 in a metabolic transformation of arachidonic acid (7, 13-15) as well as in prostaglandin ( 16) and leukotriene ( 17) metabolism has been also demonstrated.In recent years, great interest has focused on the role of metabolites related to arachidonic acid (7, 8) and some of the hydroperoxide derivatives (18-20) because these