Two major groups of renal chemosensory neural elements have been identified in the rat: one specifically activated by renal ischemia, the previously described "R" chemoreceptors, and the other by backflow of nondiuretic urine into the renal pelvis. The latter group is the object of the present investigation. In anesthetized, male Sprague-Dawley rats, single-unit recordings were obtained by dissection of the centrally cut nerves of the right kidney. The responses of single units to backflow into the renal pelvis of nondiuretic urine, diuretic urine, and solutions containing urea, mannitol, or inorganic ions were compared. The excitatory effect of the backflow of nondiuretic urine was due to its chemical composition rather than to changes in pelvic pressure and pelvic distension. The same units were activated markedly by renal ischemia. The resting discharge rate of the units was very high in nondiuretic conditions, and it declined progressively when diuresis was induced by expansion of the extracellular fluid volume. It is concluded that this group of sensory elements responds to the chemical environment in the renal interstitium as modified by ions crossing the pelvic epithelium, by leakage of ions out of ischemic cells, and by alterations in the excretory function of the kidney and renal blood flow. This group of renal sensory nerve endings has been termed "R2" chemoceptive receptors, to distinguish them from the previously described group of renal "R" chemoreceptors. Circ Res 46: 395-405, 1980