Lysine clonixinate (LC) is a non-steroidal antiinflammatory agent (NSAID) with only few adverse effects. This characteristic has prompted us to suggest that its administration, at levels equivalent to those found in human plasma following therapeutic doses, slightly inhibits cyclooxygenase I (COX I). Three experiments were performed. Experiment 1: to study the in vitro effect of LC at concentrations of 4 and 6 micrograms/ml, comparable with those found in plasma following an oral therapeutic dose of 125 mg. Gallbladder tissue segments were incubated with 0.25 microCi of 14C-arachidonic acid and the production of prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2 alpha) and 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha) was measured. LC did not affect basal production of any of the 3 prostaglandins (PGs) but at 6 micrograms/ml slightly reduced the levels of 5-hidroxyeicosatetraenoic acid (5-HETE). Experiment 2: LC was administered preoperatively to 6 patients by continuous perfusion, to achieve a steady-state concentration between 4 and 6 micrograms/ml. Gallbladder segments from the 6 treated and another 6 control patients were incubated in 14C-arachidonic acid. Gallbladder segments treated with LC did not show a decreased production of any of the three PGs whereas 5-HETE released to the medium was significantly lower. Experiment 3: 18 patients received an i.v. bolus of LC 100 mg (n1 = 6) or LC 200 mg (n2 = 6) or indomethacin (INDO) 50 mg (n3 = 6). Unlike the administration of INDO bolus, LC in the above doses did not inhibit PG synthesis. Both NSAIDs showed different effects when the production of 5-HETE synthesis was assessed. Treatment with INDO did not alter the production of 5-HETE while LC elicited significant inhibition. The three studies conducted, namely in vitro and in vivo continuous perfusion and i.v. bolus, revealed that LC had no effect on prostaglandin synthesis while reducing significantly the levels of 5-HETE.