The distribution and the ultrastructure of afferent fibers innervating the parietal peritoneum in the rat was studied with immunohistochemistry using an antiserum against the neuronal marker protein gene product 9.5. The immunoreactive fibers were distributed throughout the peritoneum. They generally ran straight and parallel to the intercostal nerves running in the abdominal muscles underlying the peritoneum. They sometimes branched and terminated by forming club-like endings. The number of nerve endings on the peritoneal surface was 3.25 ± 1.66 mm−2. Electron microscopic observations revealed both unmyelinated and myelinated nerve fibers. The unmyelinated fibers were thin and about 1 μm in diameter. Their endings formed slight swellings located just inside the peritoneal cell layer. The myelinated fibers often formed a bundle that was composed of two or three nerve fibers. Each myelinated fiber kept in contact with a Shwann cell and projected toward the peritoneal cavity. Finally, they penetrated the peritoneal cell layer to reach the peritoneal cavity. These fibers then made contacts with the peritoneal cells and became free from the myelin sheath. The ending had a club-like shape covered with collagen fibers, and contained many neurofilaments, a few mitochondria, but no synaptic vesicles. These results suggest that since the sensory endings are exposed at the peritoneal cavity, the sensory fibers are highly sensitive to somatic or nociceptive stimuli. Anat Rec, 2011. © 2011 Wiley-Liss, Inc.