Co-oxidative metabolism of ANFT by either the fatty acid cyclo-oxygenase or hydroperoxidase activities of prostaglandin endoperoxide synthetase was examined by using solubilized and particulate microsomes prepared from rabbit renal inner medulla and ram seminal vesicles. The rate of metabolism was measured by the decrease in absorbance at 385 nm. In both soluble and particulate preparations, ANFT metabolism was dependent upon specific fatty acid substrates and prevented by specific inhibitors of prostaglandin endoperoxide synthetase. Other inhibitor and substrate specificity studies suggest that ANFT was not metabolized by xanthine oxidase, lipoxygenase, lipid peroxidation, or mixed-function oxidases. Under incubation conditions which demonstrated co-oxidation of ANFT, a metabolite (peak I) was observed by high-pressure liquid chromatography. In the presence of indomethacin, peak I was not present, and only authentic ANFT was observed. Co-oxidation of ANFT was also observed with cumene hydroperoxide. Cumene hydroperoxide-mediated co-oxidation was not prevented by indomethacin or SKF-525A but was blocked by the antioxidants butylated hydroxytoluene, ethoxyquin, and vitamin E. The data indicate that ANFT is metabolized by a co-oxidative process involving the prostaglandin hydroperoxidase activity of prostaglandin endoperoxide synthetase.