Prostanoids produced via the action of cyclo-oxygenase-2 (COX-2) appear central to many inflammatory conditions. Here we show in LPS-treated rats, however, that COX-2 induction alone does not greatly increase prostanoid production in vivo. For this, a second, arachidonic acid liberating stimulus is also required. Thus, only after intravenous injection of bradykinin or exogenous arachidonic acid was a marked increase in prostanoid formation seen. There is, therefore, synergy between proinflammatory mediators: both induction of COX-2 protein and an increase in the supply of arachidonic acid are required to greatly enhance prostanoid production. Second, we show that supplying arachidonic acid to increase prostanoid production reduces the effectiveness of both currently used nonsteroidal antiinflammatory drugs (NSAIDs) (diclofenac) and novel COX-2-selective inhibitors (NS-398, celecoxib) as inhibitors of COX-2 activity. Our data lead to two important conclusions. First, increased prostanoid production in inflammation is a two-component response: increased COX-2 expression and increased arachidonic acid supply. Second, the supply of arachidonic acid to COX-2 determines the effectiveness of NSAIDs. NSAIDs and selective COX-2 inhibitors, therefore, will generally be less effective at more inflamed sites, providing a rationale for the very high doses of NSAIDs required in human conditions such as rheumatoid arthritis.--Hamilton, L. C., Tomlinson, A. M., Mitchell, J. A., Warner, T. D. Synergy between cyclo-oxygenase-2 induction and arachidonic acid supply in vivo: consequences for nonsteroidal antiinflammatory drug efficacy.