The sensory innervation of the rat kidney and ureter was investigated in wholemount preparations and sectioned materials by labeling the afferent nerve fibers with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) transported anterogradely from dorsal root ganglia. Labeled fibers were seen in large numbers in the ureter and in the lining of the renal pelvis, where they were located in the adventitia, smooth muscle, subepithelial connective tissue, and epithelium. Most of the fibers in the ureter and ureteropelvic junctional zone traveled parallel to the long axis of the organ. In contrast, fibers in the widest part of the funnel-shape renal pelvis were oriented predominantly in a circumferential fashion. Many of the pelvic afferents were extremely fine and appeared to terminate as free nerve endings. Modest networks of labeled axons were also observed around branches of the renal artery; the greatest innervation was supplied to the distal portions of the interlobar arteries and to the arcuate arteries. Only single axons were observed around the interlobular arteries, and very few fibers were seen around afferent arterioles or near glomeruli. In contrast to the arteries, branches of the renal vein were relatively sparsely innervated. Occasional labeled fibers entered the renal cortex and formed intimate associations with renal tubules; however, the vast majority of renal tubular elements were not contacted by labeled sensory fibers. Labeled fibers were never observed in the renal medulla or in the papilla. The present study represents the first time that the sensory innervation of the kidney and ureter has been investigated by using a highly specific anterograde nerve tracing technique. The pattern of innervation demonstrated here reveals an anatomical configuration of ureteral and renal pelvic sensory nerves consistent with a role in detection of ureteral and pelvic pressure and chemical changes and a renal vascular sensory innervation that may monitor changes in renal arterial and venous pressure and chemical content. Still other renal afferent nerve endings may signal renal pain.