The pharmacological action of salicylate cannot be explained by its inhibition of cyclooxygenase (COX) activity. In this report, the effects of aspirin and sodium salicylate on COX-2 expressions in human umbilical vein endothelial cells and foreskin fibroblasts were evaluated. Aspirin and sodium salicylate at therapeutic concentrations equipotently blocked COX-2 mRNA and protein levels induced by interleukin-1beta and phorbol 12-myristate 13-acetate. The suppressing effect was more pronounced in cultured cells deprived of fetal bovine serum for 24 h, suggesting that it may be cell cycle related. Salicylate inhibited nascent COX-2 transcript synthesis but had no effect on COX-2 mRNA stability. It inhibited COX-2 promoter activity in a concentration-dependent manner. In mice pretreated with aspirin (10 and 30 mg/kg), followed by challenge with lipopolysaccharide, COX-2 mRNA expression in peritoneal macrophages was markedly suppressed. These findings suggest that salicylate exerts its antiinflammatory action in part by suppressing COX-2 induction, thereby reducing the synthesis of proinflammatory prostaglandins.