The straight spermatic and highly convoluted testicular arteries were studied by light microscopy in adult and aging normotensive (NT) and spontaneously hypertensive (SHR) rats. In younger rats, on the internal part of bends of the testicular artery, areas lacking the media similar to classical cerebral arterial medial defects were observed. At the same location, in other bends, structurally defective areas (SDA) constituted by or including medial defects but also lacking the internal elastic lamina and which in some cases evaginated, were present. Structurally defective areas were less numerous in SHR than in NT rats at 6 months, suggesting that intrinsic differences may exist between rat strains. In contrast, in old rats, the number of SDA was higher in hypertensive than in normotensive rats, supporting the role of hemodynamics in SDA formation. With age, SDA enlarged in both rat strains, and most of them became structurally similar to aneurysms, ie, lacking the internal elastic lamina and medial cells and with a dilated lumen, supporting the view that medial defects are sites of aneurysmal structure development. In hypertensive rats, fibrin and lipid deposits occurred within these aneurysmal-like structures. In the straight part of the spermatic artery no such structural modifications occurred, suggesting that either hemodynamics and/or structural development, both dependent on arterial geometry, are determinant in SDA formation. The results are discussed in view of the use of the rat testicular artery as a possible model of the formation of spontaneous aneurysmal-like structures relevant to cerebral aneurysms.