Xenobiotic chemicals can be metabolized to mutagenic products by organs other than the liver. The prostaglandin endoperoxide synthetase (PES) system is found in a number of organs and is not dependent on cytochrome P-450. The metabolism of the cooked food-derived aromatic amines Trp-P-1 and Trp-P-2, and the imidazoquinolines IQ and Methyl-IQ, which are activated to mutagens for Salmonella by the cytochrome system, at concentrations below which they are direct-acting mutagens, are not activated by PES at these same concentrations.