The present study was to investigate the effect of intrathecal (i.t.) injection of interleukin-1 beta (IL-1 beta) on nociception in normal and inflammatory rats. Peripheral inflammation was induced by intraplantar injection (i.pl.) of carrageenan into unilateral hind paw. The nociceptive threshold to noxious thermal stimulation was measured by the paw withdrawal latency (PWL). Intrathecal injection of IL-1 beta (10 ng, 100 ng) significantly increased PWL in normal rats, the peak occurred at 5 min and the effect lasted for 30 min. Similarly, IL-1 beta (10 ng, 100 ng, i.t.) significantly increased the PWL and lasted for more than 60 min in inflammatory rats. Both in normal and inflammatory rats, the IL-1 beta-induced antinociceptive effect was completely abolished by IL-1ra (50 ng, i.t.), and apparently attenuated by naloxone (10 microg, i.t.) or mianserin (20 microg, i.t.). These results suggest that IL-1 beta produces antinociceptive effect by binding IL-1 receptor at the spinal level, and is related to the activation of opioid and 5-HT systems.