Introduction: Recent experiments demonstrated increased isobaric arterial compliance and distensibility in hypertensive patients, which could be explained by a decrease in arterial wall stress, when assessed at constant pressure. We investigated the effect of hypertension on the mechanical properties of the carotid arterial wall, and especially wall stress, assessed in vivo in spontaneously hypertensive rats (SHR). Methods: Right carotid artery internal diameter and wall thickness were continuously monitored in eight 30-week-old anesthetized SHR and seven normotensive Wistar-Kyoto (WKY) rat controls, using a high-resolution (1 μm) transcutaneous A-mode echo-tracking system (Nius 2; Asulab, Lausanne, Switzerland) coupled to a measurement of left carotid arterial pressure (micro-manometer). Results: Mean arterial pressure was 123±6 mmHg in WKY rats and 202±2 mmHg in SHR (P<0.01). Mechanical parameten were calculated at mean pressure from internal diameter and wall thickness pressure curves. There was no difference between the groups in internal diameter (1.129 and 1.200 mm for WKY rats and SHR respectively, NS) and midwall stress (10.5±12 and 10.5±0.7 106 dyn/cm2 for WKY rats and SHR respectively, NS); wall thickness was increased in SHR (105±8 and 191±16 μm for WKY rats and SHR respectively, P<0.01). Incremental modulus (wall rigidity) was increased in SHR (0.86±0.09 and 1.96±0.25 107 dyn/cm2 for WKY rats and SHR respectively, P<0.01), whereas disteraibility (1.77±0.16 and 0.54±0.06 10-3/mmHg, P<0.01) and compliance (1.84±0.30 and 0.61±0.07 10-3 mm2/mmHg, P<0.01) were decreased. Conclusions: Thus (1) in SHR, mid-wall stress is maintained at a level similar to mat of WKY rats, confirming previous in vitro data; (2) this maintained stress is most likely the consequence of the increased arterial wall thickness; (3) in contrast to the results obtained at constant pressure, at identical mid-wall stress, the increased modulus explains the decreased extensibility and compliance and reflects the altered intrinsic properties of the carotid arterial wall in SHR.