...The goal of the current investigation was to quantitate differences in stiffness down the aortic tree and the mechanisms mediating those differences in older, premenopausal (24 ± 0.7 yr) versus young adult (7 ± 0.7 yr) female nonhuman primates. Aortic stiffness (β), calculated from direct and continuous measurements of aortic diameter and pressure in chronically instrumented, conscious macaque monkeys, increased 2.5-fold in the thoracic aorta and fivefold in the abdominal aorta in old premenopausal monkeys. The aortic histological mechanisms mediating increased vascular stiffness, i.e., collagen/elastin ratio, elastin, and collagen disarray, and the number of breaks in elastin and collagen fibers were greater in the old premenopausal versus young monkeys and greater in the abdominal versus the thoracic aorta and greatest in the iliac artery. In addition, more immature and less cross-linked fibers of collagen were found in the aortas of young females. Aortic stiffness increased in old premenopausal female monkeys, more so in the abdominal aorta than in the thoracic aorta. Histological mechanisms mediating the increased aortic stiffness were augmented in the old premenopausal females, greater in the abdominal versus the thoracic aorta, and greatest in the iliac artery.NEW & NOTEWORTHY This is the first study to examine vascular stiffness down the aortic tree in aging premenopausal females (24 ± 0.7 yr old), whereas prior work studied mainly rodents, which are short-lived and do not undergo menopause. Histological mechanisms mediating vascular stiffness in older premenopausal females increased progressively down the aortic tree, with greater increases in the abdominal aorta compared with the thoracic aorta and with the greatest increases and differences observed in the iliac artery.