In the present study, the effect of contralateral and bilateral electrolytic lesions of the nucleus reticularis paragigantocellularis (PGi) on tonic pain was assessed in rats. Pain-related behavior was evaluated using the formalin test 7 days after the lesion was made. PGi lesions did not produce any marked changes in the early phase of the formalin test, but significantly increased pain sensitivity in the late phase compared to the sham-operated group. Furthermore, there was no significant difference between contralateral and bilateral lesions. Since PGi lesions resulted in significant hyperalgesia, we may conclude that this region of the rostroventral medulla plays a modulatory role in the regulation of tonic pain induced by formalin as a noxious chemical stimulus.