Using in vitro quantitative receptor autoradiography the present study reports on the distribution and possible changes of [125I]neuropeptide Y (NPY) binding sites in the rat spinal cord following neonatal capsaicin treatment, dorsal rhizotomy and sciatic nerve section. In control spinal cord the highest density of [125I]NPY binding sites was noticed in the superficial layers of the dorsal horn whereas low-to-moderate densities of [125I]NPY binding sites were detected in the deeper dorsal horn and in the ventral horn. In comparison with control rats, neonatally treated capsaicin rats showed a significant (P less than 0.001) bilateral decrease in [125I]NPY binding sites in the superficial layers of the dorsal horn. Unilateral dorsal rhizotomy and unilateral sciatic nerve section also exhibited a significant (P less than 0.05) depletion in [125I]NPY labeling in the superficial layers of the dorsal horn ipsilateral to the surgery. These results suggest that a certain proportion of [125I]NPY receptor sites is located on the primary afferent fibers of the superficial layers of the dorsal horn. This peptide thus could play an important role in the modulation of nociceptive transmission by acting directly on primary afferent terminals.