1. In rats anaesthetized with pentobarbitone sodium, a unilateral acute arthritis was produced by the injection of kaolin and carrageenan into one knee-joint cavity. Four hours after injection, the medial articular nerve (MAN) was sectioned distally and recordings obtained from the proximal stump of the nerve. 2. Centrifugally conducted action potentials were recorded from the cut MAN following the development of arthritis. Acute dorsal rhizotomy, but not sympathectomy, prevented the action potentials, and so it is concluded that the action potentials represent dorsal root reflexes. 3. Central administration of either the GABAA receptor antagonist, bicuculline, or the non-NMDA receptor antagonist, CNQX, also prevented dorsal root reflexes in the MAN. 4. Neither the GABAB receptor antagonist, CGP35348, nor the NMDA receptor antagonist, AP7, altered the dorsal root reflexes in the MAN. 5. It is concluded that arthritis causes excess primary afferent depolarization in the dorsal horn of the spinal cord leading to dorsal root reflexes. It is proposed that these dorsal root reflexes contribute to the inflammation.