The distensibility of the arterial system, which is partly determined by arterial wall structure, smooth muscle tone, and actual pressure level, decreases with aging and hypertension. Our aim was to compare aortic wall properties in 3- and 6-month-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) at comparable blood pressures in vivo. During ketamine/xylazine anesthesia in rats we performed ultrasound arterial wall tracking and invasive pressure measurements to determine, at the level of the thoracic aorta, diastolic pressure, diastolic lumen area, changes in pressure and lumen area during the cardiac cycle, and indexes of compliance and distensibility. These observations were combined with histological measurements for determination of media cross-sectional area and thickness and the incremental elastic modulus under conditions as expected in situ. Anesthesia abolished the difference in diastolic pressure between SHR and WKY. Between 3 and 6 months of age in WKY, diastolic area and incremental elastic modulus increased significantly, distensibility decreased, and all other recorded variables were not modified. Between 3 and 6 months of age in SHR, diastolic area and incremental elastic modulus increased, distensibility of the aortic wall decreased, and all other mechanical and structural properties did not change significantly. At both ages, diastolic area and compliance were significantly smaller in SHR than WKY. The other mechanical and structural properties measured or calculated at comparable pressure did not differ between strains. Differences between the aorta of 3- and 6-month-old rats and between strains observed in vivo at comparable pressures can largely be attributed to differences in lumen caliber.(ABSTRACT TRUNCATED AT 250 WORDS)