The projections of afferent renal fibers (ARN) to dorsal root ganglia and into the spinal cord of the rat were studied using the anterograde transport of horseradish peroxidase (HRP). Crystalline HRP was applied to the proximal cut ends of renal nerves or injected as a concentrated solution into the kidney, on either the right or left side. After a survival time of 40-120 h, sections of thoraco-lumbar dorsal root ganglia (DRG) and spinal cord were cut and processed according to the tetramethyl benzidine method. HRP applied either to the ARN or to the kidney on the left labeled neurons in the DRG from T8 to L2. On the other hand, HRP application on the right side resulted in labeling of neurons in DRG T6 to T13. No labeled neurons were found in the contralateral DRG. Labeled neurons in the DRG were of the small (11-20 micron) and medium (30-42 micron) size and were distributed in all portions of the DRG. In the spinal cord the greatest concentration of labeled ARN from the left were found in segments T10-L1, whereas projections from the right ARN were concentrated primarily in segments T7-T10. Labeled fibers entered along the medial aspect of the ipsilateral dorsal horn and projected both rostrally and caudally in the medial portion of Lissauer's tract, sending some collaterals into lamina I. The majority of labeled fibers coursed ventrally along the medial aspect of the dorsal horn towards the midline where they terminated in the region of the dorsal gray commissure, just dorsal to the central canal. Additionally, labeled fibers from the medial projection passed into laminae III-V. No labeled fibers or terminals were observed in the contralateral spinal cord. These data show that ARN enter the spinal cord through several DRG and provide the first anatomical demonstration of central sites of termination of ARN. These spinal sites of projection of sensory information from the kidney are likely to be central sites of integration of reno-renal and visceral reflexes.