Electrophysiological recordings were made from superficial parts of the spinal dorsal horn in monkeys, using dye-filled micropipette electrodes to permit iontophoretic marking of the recording sites for subsequent histological recovery. Focal field potentials and unitary activity evoked by dorsal root volleys including slowly-conducting components (both myelinated and unmyelinated) were found in the posteromarginal zone and the substantia gelatinosa (SG). Unitary potentials identified as being of the type recorded from cellular regions were separated into categories according to which group of slowly-conducting fibers and which kinds of cutaneous stimulation evoked the discharge. Recording locations for units excited by volleys in myelinated fibers conducting under 35 m/sec, by the types of skin stimulation activating either high-threshold mechanoreceptors (nociceptors) or cooling thermoreceptors, and giving no evidence of suprathreshold C-fiber excitation were centered on the posteromarginal zone. In contrast, recording loci for units exhibiting a strong C-fiber excitation and responses to cutaneous stimulation known to effectively excite C-fiber polymodal nociceptors or C-mechanoreceptors were centered in the SG. There appeared varying degrees of convergence of primary afferent input to the neuronal units, although most showed substantial specificity in their afferent excitation. On the bases of these results and consideration of existing morphological data, it is proposed that the marginal zone is a major synaptic termination region for the afferent fibers from high-threshold mechanoreceptors, cooling thermoreceptors, and perhaps other receptors with fine myelinated peripheral fibers. The SG, on the other hand, is suggested to be the terminal region for all types of unmyelinated primary afferent sensory neurons, and to have the complex role of integrating and distributing this input.