The present study localized calcitonin gene-related peptide at the light and electron microscopic levels in the lumbar spinal cord of the rat. One finding was that axons and terminals were labeled in both lamina I and Ilo medially but only in lamina I laterally. The functional implications of this innervation pattern are not clear but presumably this anatomic arrangement bears on both dorsoventral and mediolateral patterns of organization of primary afferent input into the dorsal horn. We also found that although the means of labeled myelinated and unmyelinated axon diameters in the tract of Lissauer were different, there was great overlap in these populations. Furthermore, subcellular localizations indicated that immunostaining of calcitonin gene-related peptide was associated primarily with microtubules in axons and cores of large dense-core vesicles in presynaptic terminals. Finally, labeled presynaptic terminals contained relatively few large dense-core vesicles and formed the presynaptic elements of simple axodendritic contacts almost exclusively. These last findings contrast with localizations of calcitonin gene-related peptide in the monkey, which has many more large dense-core vesicles in labeled terminals and in which a much higher proportion of labeled endings form the central parts of glomeruli.