Several recent results obtained in hypertensive animals and subjects under in vivo isobaric conditions do not confirm the classic view of stiffer arteries in hypertensive subjects. We compared the mechanical behavior of in situ isolated common carotid arteries from normotensive Wistar-Kyoto rats (WKY) and age-matched spontaneously hypertensive rats (SHR) under both static and dynamic conditions for transmural pressure ranging from 50 to 200 mm Hg. The static pressure (P)-diameter (D) relationship was shifted to higher values of diameters in the SHR mainly because of a larger unstressed carotid diameter (Do) in hypertensive rats. The carotid mechanical strain, calculated as (D-Do)/Do, was significantly reduced in SHR at pressure levels between 100 and 200 mm Hg. The static carotid compliance and distensibility were markedly smaller in SHR than in WKY carotid arteries, indicating a stiffer wall in hypertensive animals. In contrast, carotid compliance and distensibility were similar under dynamic conditions close to the in vivo pulse pressure (frequency, 300 bpm; peak amplitude of the oscillatory pressure, 20 to 25 mm Hg). However, marked differences in dynamic compliance- and distensibility-strain relationships in SHR and WKY are evidence of clearly different arterial wall material properties in both strains. We therefore conclude that larger lumen carotid arteries in hypertensive rats could compensate for a stiffer arterial wall, resulting in similar dynamic compliance and distensibility in normotensive and hypertensive rats.