Using in situ spinal dorsal horn perfusing method in decerebrated rats, we measured the release of immunoreactive substance P (iSP) in polyarthritic rats. The spontaneous release of iSP from the dorsal horn in the polyarthritic rats was significantly accelerated over that in control non-inflamed rats. Passive movements of the inflamed ankle joint evoked a significant increase in the release of iSP, while a similar movement of the non-inflamed joint led to no such increase. These results suggest that the facilitated release of iSP from the primary afferent terminals in the spinal dorsal horn in polyarthritic rats possibly plays a role in transmission of chronic pain of inflamed joints.