Diabetic patients with sensory neuropathy are predisposed to disorders of the musculoskeletal system. It has been postulated that altered neurogenic inflammation, involving the neuropeptide substance P, may play a part in this phenomenon. We investigated the effect of streptozotocin-induced (STZ) diabetes on the development of an antigenic (mBSA) monoarthritis in the rat with particular reference to changes in substance P levels in dorsal root ganglia (DRG) and knee joint synovium. We found that STZ-induced diabetes of 24 weeks duration reduced the substance P content of L4/L5 DRG and knee joint synovial tissue. Induction of mBSA arthritis in diabetic rats resulted in diminished increases in synovial substance P and knee joint swelling compared to non-diabetic arthritic controls. The results show that chronic STZ diabetes reduces neurogenic inflammatory responses in the rat knee which may render the joint more susceptible to arthritic attack.