The morphology of the mesenteric arterial wall, and the role of the arachidonate cascade in ADP-induced thromboformation, is studied in two groups of streptozotocin-induced diabetic rats, in comparison to a group of normal rats. Light microscopy demonstrates the dilated state of the arteries of the diabetic animals, together with the diminished wall thickness in "young diabetics", and an increased thickness in "old diabetics", as compared to normal animals. Scanning electron microscopy shows the area of local deendothelialization in the arteries of "young diabetics" to be larger than in both other groups. Combined use of a cyclooxygenase and a prostacyclin synthetase inhibitor, reveals that the decreased PGI2 release from arteries of diabetic animals, can be related to a diminished activity of the prostacyclin synthetase. I.