1. Modulation of prostaglandin biosynthesis in vivo by either exogenous or endogenous nitric oxide (NO) has been studied in the rat using arachidonic acid (AA)-induced paw oedema and measuring both the foot volume and the amount of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), the stable metabolite of prostacyclin (PGI2), in the oedematous fluid recovered from inflamed paws. 2. Paw injections of 150 or 300 nmol of AA were virtually inactive whereas 600 nmol produced a moderate oedema which was greatly reduced by the NO synthase inhibitor L-NG-nitro arginine methyl ester (L-NAME, 100 nmol/paw) and the NO scavenger haemoglobin (Hb, 30 mumol/paw), but unaffected by the inhibitor of the soluble guanylate cyclase, methylene blue (Mb, 3 mumol/paw) and L-arginine (15 mumol/paw). 3. The NO-donors (10 mumol/paw) 3-morpholino-sydnonimine-hydrochloride (SIN-1), S-nitroso-N-acetyl-D, L-penicillamine (SNAP) and sodium nitroprusside (SNP) significantly potentiated the paw oedema induced by AA (300 nmol/paw). 4. SIN-1 (2.5, 5 and 10 mumol/paw) produced a significant dose-dependent increase of the oedema induced by AA which was correlated with increased amounts of 6-keto-PGF1 alpha in the fluid recovered from inflamed paws. 5. Both oedema and prostaglandin biosynthesis induced by the combination AA+SIN-1 were greatly suppressed by either Hb (30 mumol/paw) or indomethacin (3 mumol/paw or 5 mg kg-1 s.c.) but unaffected by Mb (3 mumol/paw). 6. In LPS-treated rats (6 mg kg-1, i.p.) doses of AA inactive in normal animals produced a remarkable oedema which was reduced by L-NAME or Hb, unaffected by Mb and increased by L-arginine.7. These results demonstrate that NO increases prostaglandin biosynthesis in vivo through a guanosine 3': 5'-cyclic monophosphate (cyclic GMP)-independent mechanism and suggest that the interaction between NO synthase and cyclo-oxygenase (COX) pathways may represent an important mechanism for the modulation of the inflammatory response.