Levels of the endocannabinoids anandamide (AEA), 2-arachidonyl glycerol (2-AG), palmitoyl ethanolamide (PEA), and oleoyl ethanolamide (OEA) in synovial fluid of non-inflamed normal volunteers (control) (n = 6) and of patients with osteoarthritis (OA) (n = 14) and rheumatoid arthritis (RA) (n = 5). Data were analysed using the non-parametric Mann-Whitney test and are expressed as picomoles per millilitre. **P < 0.01, ***P < 0.005 compared with control values. ▽ P < 0.05 compared with OA values. ND, not detected below a limit of detection of 0.3 pmol/mL.