The fine structure of the rat renal papilla was studied after fixation with various fixative solutions and by using three modes of fixative application: immersion, vascular perfusion and injection of fixative into the renal pelvis. The morphology of the following was described: thin limbs of Henle's loop, collecting ducts, capillaries, interstitial cells, and interstitium. Several previously undescribed features were seen. Two types of capillaries were identified on the basis of their fine structure. Regions identified as limbs of Henle had at least two types of mural structure differing in degree of cellular interdigitation. In addition, numerous projections were seen extending from the basal cytoplasm of capillary endothelium and collecting duct epithelium through the basement membrane into the interstitium.