Structural changes in the ascending thoracic aorta of hereditary hypertriglyceridemic (hHTG), insulin-resistant, and hypertensive rats were studied using transmission electron microscopy. Normotensive Wistar rats were used as controls. The most-pronounced morphological changes were observed in the tunica intima. Endothelial cells of hHTG rats formed a continual layer around the whole circumference. Subendothelial space was enlarged. Some endothelial cells were delaminated from the subendothelial space by big lipid droplets that were often present in the subendothelial space, and the endothelial cells bulged out towards the lumen. Big electron-lucent lipid droplets were present in the majority of the endothelial cells and occupied the main part of the cytoplasm. Degenerative microvesicular and membranous material was present in the cytoplasm. Increased numbers of vesicles of Golgi apparatus and cisternae of endoplasmic reticulum were found. Similar morphological alterations, but in less-extended form, were observed in smooth muscle cells. The organization and orientation of smooth muscle cells were essentially intact. In muscle cells, lipid droplets were localized in close relation to Golgi complex and in dilated cisternae of the sarcoplasmic reticulum. Lipid droplets, degenerative material, myelin figures, myelinoid membranes, and vesicular components were also sporadically found in the intercellular space among muscle cells. This pilot morphological investigation provides further arguments for a thorough and more-focused electron microscopy study of conductance arteries of the hHTG rats.