Inflammatory pain, characterized by a decrease in the nociceptive threshold, arises through the actions of inflammatory mediators. Mitogen-activated protein kinase cascades participate in peripheral nociceptive sensitization. We examined the involvement of c-Jun N-terminal kinase (JNK) in the dorsal root ganglion (DRG) in the early phase of inflammation-induced hyperalgesia. An intra-plantar (i.pl.) injection of complete Freund's adjuvant induced the activation of JNK in DRG neurons within 30 min. Pretreatment as well as post-treatment of rats with a JNK inhibitor, SP600125, significantly attenuated thermal hyperalgesia, as assessed by paw-withdrawal latency, and the upregulation of c-fos immunoreactivity in dorsal horn neurons. An i.pl. injection of nerve growth factor (NGF) also induced the phosphorylation of JNK as well as thermal hyperalgesia, and SP600125 improved hyperalgesia. Inhibitor experiments suggest that JNK and extracellular signal-regulated protein kinase act on primary nociceptive neurons synergistically. These findings demonstrate that JNK is a therapeutic target for treating inflammation-induced pain hypersensitivity. (c) 2005 Elsevier Inc. All rights reserved.