In a recent hypothesis, ferrous heme is viewed as interacting with arachidonic acid to convert it to prostaglandin G2. If this hypothesis is correct it must be possible to explain how the ferric heme in hemoglobin which is usually added to the cyclo-oxygenase enzyme to restore activity is reduced. In the present paper we explore the possibility that reduction of the heme is accomplished by lipid peroxides, to see whether such an affect could explain the regulation of cyclo-oxygenase activity by "peroxide tone." Lipid peroxides formed by auto-oxidation of arachidonic acid were found to reduce ferric heme to ferrous heme. The amount of reduction of heme was proportional to the concentration of peroxide. A result of this finding is the expansion of the earlier hypothesis to understand how functional regulation of the cyclo-oxygenase activity may be achieved.