In health, PVAT contributes anti-contractile factors, such as adiponectin, that relax the artery. This benefit is lost in cardiovascular disease, including hypertension. We discovered that PVAT’s contributions to arterial function include more than production of vasoactive substances. Using isometric techniques, we tested the hypothesis that PVAT provided structural benefit to the arterial wall by assisting the function of stress relaxation (SR). Arterial rings were mounted with (+) and without (-) PVAT, from the same animal, and taken through a cumulative length tension curve, in which increasing tension (0 - 6 grams) was added. Tissues relaxed for 30 minutes prior to measuring the tension to which the tissue relaxed. Tissues were then challenged with phenylephrine (10 -5 M) for viability at each tension. In thoracic aorta isolated from normal male Sprague-Dawley rats, the presence of PVAT (a brown fat) along the vessel wall assisted SR by reducing the maximum (6 g) cumulative tension achieved (in mgs; -PVAT = 4578±190; +PVAT = 2730±274, p<0.05). Similarly, the PVAT in the superior mesenteric artery (a white fat) promoted SR in a cumulative length tension curve, resulting in a final tension reduced 19% compared to rings -PVAT. The physical association of PVAT with the vessel wall was essential; when PVAT was left only attached to the ends of the aortic rings, it provided no benefit in cumulative SR (+/-PVAT = 4122±176). Thus, passive secretion of anticontractile factors does not contribute to stress relaxation. Rather, PVAT must itself be stretched to result in production of humoral factors and/or it alone provides a mechanical benefit. Importantly, PVAT-assisted SR was lost in the thoracic aorta of the hypertensive DahlS rat (systolic BP = 175±9 mm Hg at 17 weeks) fed a high fat diet for 21 weeks (HF -PVAT = 4218±95; +PVAT = 4135±232; p>0.05) when compared to their controls (Con - PVAT = 4422±85; +PVAT = 3706±165; p<0.05; SBP = 111 ± 7 mm Hg). These important findings change how a blood vessel must be considered. PVAT is an essential functional element that should be considered in health and disease.