OBJECTIVES--In rats with adjuvant arthritis measurements were taken of the urinary excretion of nitrate, reflecting endogenous nitric oxide (NO) formation, and cyclic guanosine monophosphate (cGMP). METHODS--Urinary nitrate was determined by gas chromatography, cGMP by radioimmunoassay. RESULTS--A significant (p < 0.001), more than three fold increase of urinary nitrate excretion was found in rats 20 days after induction of adjuvant arthritis compared with non-arthritic rats. There was no significant difference in urinary cGMP excretion between arthritic rats and control animals. CONCLUSION--The data suggest that the dramatic increase of urinary nitrate excretion is due to increase of NO synthesis by the inducible form of NO synthase.