The cytokine IL-1b plays a role in rheumatoid arthritis where it contributes to the inflammatory process and the destruction of cartilage. Here we tested whether it has also the potency to induce nociception in a normal joint. Healthy adult WISTAR rats were anesthetized with sodium thiopentone (100 mg/kg, i.p.). The exposed knee joint was mechanically stimulated by innocuous (20 mNm) or noxious (40 mNm) rotations of the lower leg against the fastened femoral bone for 15 s each and nerve action potentials were counted. Nerve fibers were classified as C- or as Ad-fibers by their conduction velocity (<1.4 m/s or <10 m/s, respectively). Hundred ml of the compounds were injected into the joint cleft. C-fiber responses were significantly enhanced by 10 ng IL-1b (net increase to innocuous stimulation by 44.1±33.1 APs/15 s, to noxious stimulation by 112.0±35,9 APs/15 s within three hours, n=7 fibers) but not by a dose of 1 ng of IL-1b. The dose of 10 ng IL-1b significantly reduced Ad-fiber responses (net decrease to innocuous stimulation by -39.5±26.3 APs/15 s, to noxious stimulation by -159.8±53.2 APs/15 s within three hours, n=8 fibers). The intraarticular application of the solvent alone did not change the responses of Ad-fibers in the same time range. The intraarticular application of 100 ng of IL-1b had similar effects as 10 ng in the first hour after injection, but compromised cardiovegetative regulation. Thus we conclude that IL-1b sensitizes C-fibers, but interestingly Ad-fibers were rather desensitized.