It has been suggested that hyperexcitability in dorsal root ganglion (DRG) neurons due to altered sodium channel expression contributes to some chronic pain syndromes. To understand the role of the voltage-gated sodium channel α-SNS in inflammatory pain, we investigated the expression of α-SNS mRNA and tetrodotoxin-resistant (TTX-R) sodium current in small DRG neurons, which include nociceptive cells, following injection of carrageenan into the hind paw of the rat usingin situhybridization and patch-clamp recording. α-SNS mRNA expression in DRG neurons projecting to the inflamed limb was significantly increased 4 days following carrageenan injection, compared with DRG neurons from the contralateral side or naive (uninjected) rats (mean ± s.d. optical density ratio: ipsilateral/contralateral, 1.77 ± 0.17; ipsilateral/naive, 1.88 ± 0.36). The amplitude of the TTX-R sodium current in small DRG neurons projecting to the inflamed limb was significantly larger than on the contralateral side 4 days post-injection (31.7 ± 3.3vs20.0 ± 2.1 nA). The TTX-R current density was also significantly increased. These results demonstrate the increased expression of α-SNS sodium channels in small DRG neurons following injection of carrageenan into their projection field, and suggest that α-SNS is involved in the development of hyperexcitability associated with inflammation.