Experimental results suggest that substance P (SP) may play an important role in pain and inflammation in rheumatic diseases. Measurements of SP-like immunoreactivity (SPLI) were performed in synovial fluid (SF) and synovial tissue from 40 patients with rheumatoid arthritis (RA) or osteoarthritis (OA). High levels of SPLI were found in the SF of patients with RA compared with OA. Conversely, SPLI content in synovial tissue was higher in OA than in RA, suggesting that there is an active secretory process of SPLI into the SF in RA, thus depleting SPLI stores in the synovium. Our data support the involvement of SP in the perpetuation and exacerbation of inflammation in RA, and may also explain some clinical features of this disease.