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The correlation between endogenous nitric oxide (NO) generation and prostaglandin biosynthesis was studied in rat carrageenin pleurisy induced by the injection of 0.2 mL of 1\% lambda-carrageenin into the pleural cavity. The pleural exudate was collected at 4 hr and the amounts of NO2- + NO3- (NOx) and prostaglandin E2 (PGE2) measured. The NOx present in the inflammatory exudate was determined by measuring the NO2- with the Griess reaction, after the reduction of NO3- to NO2- using acid-washed cadmium powder. PGE2 was measured by radioimmunoassay. The NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 1-3-10 mg/kg subcutaneously) reduced NOx by 20 +/- 7\%, 41 +/- 6\% and 55 +/- 9\% (P < 0.01) and PGE2 by 9 +/- 6\%, 41 +/- 11\% and 74 +/- 9\% (P < 0.001). Conversely, L-arginine (300 mg/kg SC) increasedNOx by 39 +/- 7\% (P < 0.01) and PGE2 by 78 +/- 6\% (P < 0.001). The NO scavenger haemoglobin (Hb), coinjected into the pleural cavity (3 mg/site) with carrageenin, produced a parallel inhibition of NOx (65 +/- 16\%, P < 0.001) and PGE2 (71 +/- 18\%, P < 0.001). The soluble guanylate cyclase inhibitor methylene blue (Mb; 2 mg/site) had no effect. Moreover haemoglobin, but not methylene blue, was able to significantly suppress the L-arginine-induced increase of both NOx and PGE2. In each pleural exudate, independently from the animal treatment, the amount of NOx was highly correlated to the amount of PGE2 (r = 0.93, P < 0.001). These results suggest that in rat carrageenin pleurisy the modulation of the L-arginine:NO pathway results in a parallel modulation of prostaglandin biosynthesis. The interaction between cyclooxygenase and the NO pathway may represent an important mechanism for the modulation of the inflammatory response.