Aortic pulse wave velocity (aPWV), a major prognostic indicator of cardiovascular events, may be augmented in hypertension as a result of the aorta being stretched by a higher distending blood pressure or by a structural change. We used a novel technique to modulate intra-thoracic pressure and thus aortic transmural pressure (TMP) to examine variation of intra-thoracic aPWV with TMP in hypertensive (n=20, mean±SD age 52.1±15.3 years, BP 159.6±21.2/92.0±15.9 mmHg) and normotensive (n=20, age 55.5±11.1 years, BP 124.5±11.9/72.6±9.1 mmHg) subjects. aPWV was measured using dual Doppler probes to insonate the right brachiocephalic artery and aorta at the level of the diaphragm. Resting aPWV was greater in hypertensive compared to normotensive subjects (897±50 cm/s versus 784±43 cm/s, P<0.05). aPWV was equal in hypertensive and normotensive subjects when measured at a TMP of 96 mmHg. However, dependence of aPWV on TMP in normotensive subjects was greater than in hypertensive subjects (9.6±1.6 versus 3.8±0.7 cm/s per mmHg increase in TMP respectively, means±SEM, P<0.01). This experimental behaviour was best explained by a theoretical model incorporating strain induced recruitment of stiffer fibres in normotensive subjects and fully recruited stiffer fibres in hypertensive subjects. These results explain previous contradictory findings with respect to isobaric aPWV in hypertensive compared to normotensive subjects. They suggest that hypertension is associated with a profound change in aortic wall mechanical properties possibly due to destruction of elastin leading to less strain induced stiffening and predisposition to aortic dissection.