The endothelium serves many functional roles, including the modulation of vascular smooth muscle tone through the release of vasoactive agents such as nitric oxide (NO) and the eicosanoids. We proposed that NO produced by endothelial cells would increase the production of eicosanoids through enhanced expression and/or activation of prostaglandin H synthase. NO and eicosanoid synthesis were stimulated in a bovine coronary microvessel endothelial cell line with the calcium ionophore A23187 (1 mumol/L). Our data demonstrated the following: (1) A23187 stimulated NO synthesis along with prostacyclin and thromboxane production. (2) Inhibition of NO synthesis with NG-nitro-L-arginine methyl ester (0.1 mmol/L) significantly diminished both prostacyclin and thromboxane production. (3) Cells incubated with hemoglobin (2 micrograms/mL), which inactivates NO, decreased A23187-stimulated prostacyclin production, whereas cells incubated with superoxide dismutase (20 U/mL), which protects NO from superoxide anions, enhanced prostacyclin production. (4) Exogenous NO stimulated prostacyclin production. (5) The interaction of NO with prostacyclin persisted in the presence of excess exogenous arachidonic acid (100 mumol/L). (6) Cyclooxygenase activity in cell lysates increased in the first hour of NO stimulation. (7) NO stimulation of prostacyclin occurred within 1 hour and continued for 8 hours. (8) Neither constitutive nor inducible prostaglandin H synthase enzyme expression was altered by NO. (9) Cycloheximide (10 mumol/L) had no effect on A23187 stimulation of prostacyclin production. (10) Exogenous cGMP (10 mumol/L) or a phosphodiesterase inhibitor (1 mmol/L) did not affect prostacyclin production. These data indicate that stimulating synthesis of endogenous NO in cultured endothelial cells increased eicosanoid production through activation of prostaglandin H synthase.