Marked morphologic changes were seen in all structural elements of the renal papilla during mercuric chloride-induced acute tubular necrosis. Expect for the descriptions of increased numbers of leukocytes within the medullary vasa recta, papillary changes have not been described previously. The techniques used in this study included light microscopy, scanning electron microscopy, and transmission electron microscopy. The renal interstitial cells developed an extremely irregular contour and were penetrated by large irregular cavelike structures. A marked decrease occurred in the size and number of the cytoplasmic lipid droplets in these cells. The volume of the Golgi area, and of the rough and smooth endoplasmic reticulum increased. The morphologic changes in the renal interstitial cells were indicative of increased activity rather than of cellular injury. This is of insterest because several investigators have postulated that the interstitial cells are the site of renal prostaglandin synthesis Incrreased prostaglandin synthesis and release have been proposed to play a role in the redistribution of renal blood flow seen during acute renal failure.