Prostaglandins are pro-inflammatory but are gastroprotective. The gastric mucosa synthesizes prostaglandins mainly via constitutive cyclooxygenase (COX-1), whereas leucocytes have inducible enzyme (COX-2). Nimesulide (CAS 51803-78-2) differentially inhibited prostanoid synthesis in these human tissues as well as with in vitro enzyme assays, and was less potent than indometacin (CAS 53-86-1) on COX-1. Fresh human gastric mucosa was cut finely, washed and pre-incubated (100 mg in 1 ml phosphate buffered saline pH 7.4) with or without nimesulide or indometacin (0.1-100 micrograms/ml; 0 degree C; 30 min). The fluid was replaced with fresh identical solution, incubated (37 degrees C; 30 min) and the solution assayed. Isolated leucocytes from human peripheral blood were incubated (1-1.5 x 10(6), 2 ml Krebs' solution) with or without nimesulide or indometacin (0.1-100 micrograms/ml; 37 degrees C; 1 h), stimulated with lipopolysaccharide (5 micrograms/ml), further incubated for 24 h at 37 degrees C and the medium assayed for the prostanoids PGE, TXB2, 6-keto-PGF1 alpha and the leukotriene LTB4 by radioimmunoassay (RIA). In vitro assays with COX-1 from ram seminal vesicles, or COX-2 from sheep placenta, were performed by pre-incubating the enzymes with vehicle alone (controls) or with drug for 5 min at 37 degrees C. Arachidonate (10 mumol/l) was added and further incubated for 2 min at 37 degrees C. Reactions were terminated and PGE determined by RIA. Both drugs caused concentration-related inhibitions of prostanoid accumulation in incubates of both tissues. Nimesulide reduced PGE accumulation more potently in incubates of stimulated leucocytes than of gastric mucosa. With gastric tissue, nimesulide was less potent than indometacin by approximately 6-22 fold (IC50 for PGE, TXB2, 6-keto-PGF1 alpha, respectively; 14.8 vs 2.5; 12.8 vs 1.0; 31.1 vs 1.4 mumol/l; p < 0.05 to 0.02). With the leucocytes, the concentrations of both drugs, particularly indometacin were not low enough to calculate the IC50. With the in vitro assay, nimesulide (0.01 to 100 mumol/l) did not inhibit PGE formation by COX-1 but caused a concentration-related inhibition of PGE formation by COX-2 (4-60%). These results are consistent with the effective analgesic/anti-inflammatory activity of nimesulide coupled with better gastric tolerance compared to indometacin.