The distribution of terminals arising from dorsal root primary afferents was examined in the lumbar spinal cord of cats using light- and electron-microscopic autoradiography. Tritiated proline or leucine was injected into either the L6 or L7 dorsal root ganglion. The light-microscopic spinal cord distribution of radioactivity in the ganglia was independent of the type of amino acid used. Likewise, the length of the survival time after injection had no effect. The projections to the substantia gelatinosa and the marginal zone were consistently densest. However, the topography of the dorsal horn distribution, relative to the segment of entry, varied significantly especially in the gelatinosa, depending upon the ganglia injected. Those to the substantia gelatinosa were largely limited to the segment of entry; those to the marginal zone and nucleus proprius extended many segments beyond the level of entry. At all levels the projection was exclusively ipsilateral to the side of injection. At the electron-microscopic level the distribution of radioactivity was determined in each of the three easily recognizable areas of the dorsal horn: the marginal zone, the substantia gelatinosa and the nucleus proprius. In each dorsal horn area the total terminal population was divided into four basic categories. Each of these areas was found to contain a characteristic distribution of these four terminal categories. The difference between areas arose, primarily, as a consequence of the dorsal to ventral decreasing frequency gradient of two types of terminal: those containing large, dense-cored vesicles and the increasing gradient of those containing flattened vesicles. The terminals with small pleomorphic vesicles and those with large round vesicles were frequently encountered in all three areas without a detectable frequency gradient. Similarly the primary afferent terminal population that is the subset of the total terminal population labelled after dorsal root ganglion injection, was also characteristic of the area, and each area was dominated by a different terminal-type. In the marginal zone the terminals containing large dense-cored vesicles dominated. In the substantia gelatinosa the terminals with pleomorphic vesicles (which included the so-called 'C' type terminals) dominated. And the terminals containing large round vesicles dominated the primary afferent population in the nucleus proprius. The terminals containing flattened vesicles were never found to be specifically labeled in any of the areas examined.