There is no literature report of a detailed morphologic study of the aortic depressor nerve. The aim of this study was to describe the general morphological aspects and to obtain morphometric parameters for the aortic depressor nerve of normotensive Wistar rats (n=12). Before the morphologic studies, nerves were isolated and pressure-nerve activity curves were obtained. Basal mean arterial pressure was 117+/-5 mm Hg, the systolic pressure threshold was 100+/-7 mm Hg, and mean arterial pressure at 50% of maximal activity was 115+/-5 mm Hg and the baroreceptor gain 1.99+/-0.09%/mm Hg. Semithin and thin sections of proximal and distal nerve segments were then examined by light and electron microscopy, respectively. The main nerve components were (1) unmyelinated and myelinated axons; (2) Schwann cells; (3) capillary wall endothelial cells and pericytes; (4) collagen fibers in the epineurium and endoneurium and between perineurial cell layers; and (5) fibroblasts and mast cells. The depressor nerves were found to contain 204-996 axons per nerve, 80% of which, on average, were unmyelinated, with a 4:1 unmyelinated/myelinated axon ratio. The unmyelinated axon histogram was unimodal, with a mean diameter of 0.5+/-0.02 microm. Myelinated fibers had axons averaging 1.3+/-0.06 microm in diameter and representing 53% of the total fiber diameter. The ratio between axonal and total fiber diameter of myelinated fiber ranged from 0.4 to 0.8 and tended to increase with axon size. Proximal and distal segments were morphologically similar. In conclusion, the morphologic description of the depressor nerve provides important data for further investigations of the structural basis of altered baroreflex responses in conditions such as arterial hypertension, aging, atherosclerosis, and peripheral neuropathies.