Umbilical vessels of spontaneously hypertensive rats, control strain Wistar-Kyoto rats and rats treated with alpha-methyldopa were compared using the scanning electron microscope and light microscope. Observations with the scanning electron microscope revealed that the venous endothelial cells were relatively flat, giving the luminal surface of the vein a smooth appearance. The nuclear region of the fusiform arterial endothelial cells was responsible for the bumpy appearance of the luminal surface of the artery. Microvilli were a consistent feature of the endothelium in both umbilical vessels. There was no consistent pattern of distribution or density of microvilli within either vessel, but microvilli were more abundant on the luminal surface of the artery than in the vein. The luminal surface of some endothelial cells of the artery had long straight processes which crossed several cells before terminating. Light microscopic observations revealed that the endothelial cells and cells of the tunica intima and media contained an abundance of glycogen. The same layers stained sparsely for acid glycosaminoglycans. Maternal hypertension and treatment of spontaneously hypertensive rats with the antihypertensive drug, alpha-methyldopa, did not result in significant morphological alterations of either the endothelium or tunica media of the umbilical blood vessels.