To investigate the role of tumor necrosis factor-α(TNF-α) in the brain in nociception, we injected recombinant human TNF-α(rhTNF-α; 1 pg–10 ng/rat) into the lateral cerebroventricle (LCV) in rats and observed the changes in paw withdrawal latency to radiant heat by using the plantar test for 90 min after injection. LCV injections of TNF-αat doses of 10 pg, 100 pg and 1 ng reduced paw withdrawal latency, showing a maximal response at a dose of 10 pg which peaked 60 min after injection. TNF-αat doses of 1 pg and 10 ng had no effect on nociception during the test period. The TNF-α(10 pg)-induced reduction in paw withdrawal latency was blocked by simultaneous injection of diclofenac (1 ng), a cyclooxygenase inhibitor, or interleukin-1 receptor antagonist (IL-1ra, 10 ng). LCV injection of neither diclofenac (1 ng) nor IL-1ra (10 ng) had any effect on nociception by itself. The results suggest that TNF-αin the brain induces thermal hyperalgesia and that the brain TNF-α-induced hyperalgesia is mediated by the central action of interleukin-1 and activation of the cyclooxygenase pathway of the ar