Abstract
Deposition of bilirubin at the tip of the renal papilla has been studied in the infant homozygous Gunn rat. the deposits form during the 4th wk of life in ground substance along basement membranes of capillaries and tubules and along the borders of interstitial cells. Deposits also form in capillary lumens. the early ultrastructural appearances indicate that uncon-jugated bilirubin is not directly cytotoxic. This conclusion has important implications for the pathogenesis of enhanced nephrotoxicity in the homozygous Gunn rat and ultimately for the pathogenesis of analgesic nephropathy.