The renal inner medulla may be exposed to high concentrations of organic compounds which are excreted into the urine. This report examines the capacity of the inner medulla to metabolize organic compounds both in vitro and in vivo. The compounds used were 1,3-diphenylisobenzofuran (DPBF), luminol, and benzidine. The inner medulla was shown to possess the capacity to oxidize each of these compounds. Microsomal oxygenation did not require NADPH. Cytochorome P-450 inhibitors carbon monoxide and metyrapone did not reduce DPBF metabolism. Lipoxygenase activity was not detected in inner medullary microsomes. Oxygenation of DPBF was demonstrated in inner medullary slices and was inhibited by indomethacin. The product of DPBF metabolism in renal slices and microsomes was identified as O-dibenzoylbenzene. In vivo experiments demonstrated benzidine metabolism, which was blocked by meclofenamic acid. On the basis of substrate specificity and inhibitor studies, it was concluded that oxygenation of DPBF, luminol, and benzidine was mediated by prostaglandin cyclooxygenase. These results are compatible with cooxygenation being a mechanism of inner medullary drug metabolism.