Signal induction and molecular mechanisms of the thrombin induced prostaglandin I2 (PGI2) synthesis in human umbilical vein endothelial cells (HUVEC) were investigated in association with the intra Ca2+ and the gene expression of prostaglandin H2 synthase (PGHS) and phospholipase A2 (PLA2) using the method of competitive polymerase chain reaction (PCR). Thrombin enhanced the synthesis of PGI2 by time dependent manner. Thrombin stimulation caused an elevation of the cytosolic Ca2+, which in turn stimulate phospholipase A2 (PLA2), resulting in arachidonic acid cleavage from membrane phospholipids and its subsequent conversion into PGI2 through PGHS pathway. The elevated cytosolic Ca2+ resulted from mainly the influx of extracellular Ca2+. Compatible to former reports, PGHS-1 mRNA was constitutively expressed, while PGHS-2 and PLA2 mRNA was not. With the stimulation of thrombin, PLA2 mRNA increased to 9-fold in 15 minutes, PGHS-1 mRNA to 3.4-fold in 180 minutes, PGHS-2 mRNA to 38-fold in 60 minutes. These results suggest that PGHS-1 is not only the constitutive enzyme but also the inducible enzyme and expression of PLA2, PGHS-1, PGHS-2 mRNA cause to PGI2 generation.