During the metabolism of arachidonic acid by prostaglandin H synthase many chemicals including carcinogens are metabolized. These chemicals are metabolized by either the peroxidase activity of prostaglandin H synthase, the peroxyl radicals generated during arachidonic acid oxygenation, or a combination of these two mechanisms. In many cases, the chemical metabolism results in the formation of reactive metabolites that have mutagenic activity and potential carcinogenic activity. In other cases, the chemicals are detoxified. Chemical metabolism that occurs during arachidonic acid oxygenation may be an important determinate of chemical toxicity in extra-hepatic tissues.