Cyclooxygenase-1 (Cox-1) and Cox-2 are key enzymes in the conversion of arachidonic acid to prostaglandins and other eicosanoids. We studied the effects of plasma HDL and LDL on the synthesis of prostacyclin, Cox-1/Cox-2 mRNA, and protein expression by rabbit aortic smooth muscle cells. Prostacyclin synthesis was measured by enzyme immunoassay (EIA) of the stable metabolite of prostacyclin (PGI₂), 6-keto-prostaglandin F_1α. HDL (150 μg/mL) induced release of PGI₂ to values 3.46±0.3-fold above control. Incubations with LDL did not induce release of PGI₂. N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide (NS-398), a selective irreversible Cox-2 inhibitor, blocked the HDL-induced PGI₂ synthesis. Cycloheximide, actinomycin D, and dexamethasone downregulated HDL-induced PGI₂ synthesis; therefore, HDL induced de novo synthesis of protein and Cox-2 mRNA. In addition, Northern blot analyses did not reveal differences in Cox-1 mRNA levels between control and HDL-treated cells, whereas Cox-2 mRNA levels were significantly increased in treated cells. Western blot analysis also showed an increase in the levels of Cox-2 protein. Therefore, the effects of HDL on PGI₂ synthesis are mediated via upregulation of Cox-2 expression.