SENSITIZATION OF NOCICEPTIVE C-FIBERS 99 ample, the response to the second 45OC stimulus of the TTS in the first run was 3 1 t 5% of the response evoked by the first 45 C stimulus within the same run. 9. The conduction velocity and receptivefield size of the H-CMHs and G-CMHs did not differ. However, the mechanical threshold of the H-CMHs (2.5 1 t 0.11 bars) was significantly less than that of the G-CMHs (4.27 t 0.32 bars, P< 0.001). IO. It is concluded that H-CMHs and G-CMHs differ significantly with regard to the propensity to sensitize to noxious heat stimuli. H-CMHs sensitize readily, and G-CMHs given the same stimuli do not sensitize. These results suggest that C-fiber nociceptive afferents do not play an important role in hyperalgesia in glabrous skin but may play an important role in hyperalgesia in hairy skin. This finding also supports the hypothesis that A-fiber nociceptive afferents play an important role in explaining the marked hyperalgesia that is produced by a substantial thermal injury (53OC for 30 s) to the glabrous skin of the hand (22).