On experimental portal vein replacement of mongrel dogs with high porosity EPTFE grafts (fibril length: 60 microns) wrapped in omental pedicle flap, it has been reported that healing process is promoted. But it is not clear on the antithrombotic properties of the newly growing endothelial cells on prosthetic vascular grafts. PGI2 and NO are known as antithrombotic factors that restrain the agglutination of platelets. We evaluated PGI2 and NO production from the newly growing endothelial cells on prosthetic vascular grafts. High porosity EPTFE grafts were implanted at the portal veins of mongrel dogs. After 3 months we removed the grafts and tried to procure the new growing endothelial cells by trypsinization and culture them. We confirmed by using Dil-Ac-LDL that endothelial-like cells on prosthetic vascular grafts were as same as the endothelial cells. As control cells, we used the endothelial cells of canine portal vein and infra vena cava. The basal PGI2 production from each cells and the thrombin-stimulated production of PGI2 were measured by radioimmuno assay. Thrombin concentration was established at 2 unit/ml. On the basal PGI2 production there was no significant difference between the grafts and the portal veins, and between the grafts and the infra vena cava. The PGI2 production of the grafts increased significantly by stimulating with thrombin. On the thrombin-stimulated production, there was no significant difference between the grafts and the portal veins, and between the grafts and the infra vena cava. NO production of the grafts was measured by the griess reaction. On NO production there was no significant difference between the grafts and the portal veins. But the grafts produced significantly more NO than the infra vena cava (P < 0.05). This study suggests that the newly growing endothelial cells on the prosthetic vascular grafts may have the antithrombotic properties equal to the endothelial cells on the portal vein and the infra vena cava.