Upon noxious stimulation, substance P (SP) is released from primary afferent fibers into the spinal cord where it interacts with the SP receptor (SPR). The SPR is located throughout the dorsal horn and undergoes endocytosis after agonist binding, which provides a spatial image of SPR- containing neurons that undergo agonist interaction. Under normal conditions, SPR internalization occurs only in SPR+ cell bodies and dendrites in the superficial dorsal horn after noxious stimulation. After nerve transection and inflammation, SPR immunoreactivity increases, and both noxious as well as nonnoxious stimulation produces SPR internalization in the superficial and deep dorsal horn. We investigated the primary afferent fibers that contribute to enhanced SPR internalization in the spinal cord after nerve transection and inflammation. Internalization evoked by electrical stimulation of the sciatic nerve was examined in untreated animals, at 14 days after sciatic nerve transection or sham surgery and at 3 days after hindpaw inflammation. Electrical stimulation was delivered at intensities to excite Aβ fibers only, Aβ and Aδ fibers or A and C fibers as determined by the compound action potential recorded from the tibial nerve. Electrical stimuli were delivered at a constant rate of 10 Hz for a duration of 5 min. Transection of the sciatic nerve and inflammation produced a 33.7 and 32.5% increase in SPR and immunoreactivity in lamina I, respectively. Under normal conditions, stimulation of Aδ or C fibers evoked internalization that was confined to the superficial dorsal horn. After transection or inflammation, there was a 20-24% increase in the proportion of SPR+ lamina I neurons that exhibited internalization evoked by stimulation of Aδ fibers. The proportion of lamina I SPR+ neurons that exhibited internalization after stimulation of C-fibers was not altered by transection or inflammation because this was nearly maximal under normal conditions. Moreover, electrical stimulation sufficient to excite C fibers evoked SPR internalization in 22% of SPR+ lamina III neurons after nerve transection and in 32-36% of SPR+ neurons in lamina III and IV after inflammation. Stimulation of Aβ fibers alone never evoked internalization in the superficial or deep dorsal horn. These results indicate that activation of small-caliber afferent fibers contributes to the enhanced SPR internalization in the spinal cord after nerve transection and inflammation and suggest that recruitment of neurons that possess the SPR contributes to hyperalgesia.