To assess the roles of prostacyclin (PGI2) synthase in vascular PGI2 generation, the enzymatic activity was examined in the mesenteric artery of Dahl S rats in the prehypertensive or hypertensive stage. Elevation of blood pressure in Dahl S rats was accompanied by an increase of vascular PGI2 synthase activity. The enzymatic activity was positively correlated to the blood pressure value. Similarly, impaired vascular capacity to generate PGI2, which was observed in prehypertensive Dahl S rats, was restored to the control level of Dahl R rats with the elevation of blood pressure. These data indicate that vascular PGI2 synthase activity is increased with blood pressure elevation, thereby contributing to the restoration of vascular capacity to generate PGI2 in Dahl S rats.