The detailed morphology of the renal nerves in mice has not been reported previously. The aims of this study were to describe the general morphology of the extrinsic renal nerve in C57BL/6 mice, and determine its morphometric parameters. The major renal nerve innervating the left kidney was isolated in five mice. Thin sections of the nerve segments were then examined by transmission electron microscopy. The renal nerve averaged 35.4 +/- 3.6 (S.E.M.) microm in diameter and 741 +/- 104 microm in area. The renal nerve contained an average of 830 +/- 169 unmyelinated fibers and only 4.6 +/- 1.7 myelinated fibers. The axon diameter of myelinated and unmyelinated fibers averaged 2.2 +/- 0.3 microm and 0.76 +/- 0.02 microm, respectively. The diameter of the unmyelinated fibers ranged from 0.3 to 2.0 microm, and the distribution histogram was unimodal. The majority of fibers (85%) had diameters of 0.6-1.0 microm. These results are similar to those obtained for renal nerves of rats with respect to the predominance of unmyelinated fibers. However, the diameter of unmyelinated fibers is larger in rats and the distribution histogram of rat unmyelinated fibers is bimodal, in contrast to the unimodal distribution in mice. The morphological description of the renal nerves in mice provides baseline data for further investigations of the structural basis of altered autonomic reflexes. The results will be useful in analyses of genes that influence the development and structure of sympathetic and sensory innervation of the kidney in genetically manipulated mice.