The spinal segmental distribution and intersegmental course of primary afferent fibers were studied by injecting (by pressure or iontophoresis) tritiated amino acids (L-proline or L-leucine) into spinal ganglia of coccygeal and sacral segments and autoradiographically analyzing histological sections of the spinal cord, particularly those regions lying dorsal to the central canal. The results from eight cats and three monkeys are described. A heavy projection of primary afferent fibers to the marginal zone (lamina I), the substantia gelatinosa Rolandi (lamina II), and throughout the nucleus proprius (laminae III-IV) was demonstrated. The projections to these three areas appeared to be substantially independent. Primary afferent fibers were found to course rostrally and caudally within the marginal zone, in the midline dorsal to the central canal, in Lissauer's tract, and in the dorsal columns. A crossed projection passed by way of the dorsal commissure to the contralateral marginal zone and to a region ventrolateral to the contralateral nucleus proprius. There was a distinct medial-to-lateral shift in the termination of primary afferent fibers in the substantia gelatinosa and in the dorsal portion of the nucleus proprius. The most medial distribution occurred immediately caudal to the entry zone of the primary afferent fibers, and the most lateral at the cephalad end of the segment immediately rostral to the entry level. Small (iontophoretic) injections revealed circumscribed fields of termination, approximately 40 micrometers by 70 micrometers (dorsoventrally) by 400 micrometers or more (rostrocaudally) in the substantia gelatinosa.