Circulating human neutrophil granulocytes (PMNs) from patients with rheumatoid arthritis (RA) have earlier been described to possess an enhanced capacity for production of certain 5-lipoxygenase-derived metabolites of arachidonic acid (AA), 5-hydroxyeicosatetraenoic acid (5-HETE) and leukotriene B4 (LTB4). In the present investigation the endogenous AA metabolism of synovial fluid PMNs of RA patients was studied and compared with that of the corresponding circulating PMNs. Synovial fluid PMNs prelabelled with 14C-AA released significantly less radioactivity than circulating PMNs when stimulated with calcium ionophore. Furthermore, synovial fluid PMNs produced significantly smaller amounts of both 5-HETE and LTB4 than circulating PMNs from the same patients, whereas no such difference was observed in the LTB4 catabolites or the cyclo-oxygenase products. More information dealing with the complex way in which arachidonic acid is metabolized in diseased RA joints may provide future rational approaches in the treatment of this chronic inflammatory disease.