The elastic properties of the thoracic aorta are well known, but this is the first study of the inherent strength of the tunica media. The latter is crucial to understand how dissecting aneurysms occur. Pressure-volume (P-V) measurements were recorded as a dilute suspension of India ink was infused into the tunica media using a constant flow pump (0.21 or 0.88 ml min-1) attached to a 20 G needle inserted into the media. Tests done on 31 opened pig upper descending thoracic aortas showed the peak pressure to tear the media averaged 77.2 +/- 1.5 kPa (579 mm Hg). The initial slope of the P-V curve revealed the average distensibility of the media of 3.02 +/- 0.28 (MPa)-1. The work per unit area of tissue required to propagate a tear in the aorta was 15.9 +/- 0.9 mJ cm-2. These values were independent of the tear depth at the 95% confidence interval.