Weanling male SpragueDawley rats were randomly fed a control diet (AIN-93) (C) or a blueberry diet (B) for 13 weeks, or a reverse diet (R) (C diet for 13 weeks, switched to the B diet for 8 weeks). Aortas were excised, and two intact and two endothelium-denuded rings were immersed in tissue baths containing physiological salt solution at 37°C and aerated with 95% O2 and 5% CO2 (pH 7.4). Following equilibration and preconditioning under 1.5-g preload, cumulative doseresponse curves were generated with six doses of the 1-adrenergic receptor-selective agonist L-phenylephrine (L-Phe, 1083 106 M) and relaxed with one dose of acetylcholine (3 106 M) to assess intact endothelium. The maximum force of contraction (Fmax) and vessel sensitivity (pD2) were determined in intact and endothelium-denuded rings. A two-way analysis of variance test revealed that blueberry-fed animals (B and R diets) developed a significantly lower F max (0.873 ± 0.0463 and 0.9266 ± 0.0463 g, respectively) when contracted with L-Phe, compared with the animals on the C diet (1.109 ± 0.0463 g) (P < .05). The pD2 of the intact rings was not significantly different among diet groups. Additionally, diet did not significantly affect the mean F max or pD2 of endothelium-denuded rings. Our results indicate for the first time that wild blueberries incorporated into the diet affect the vascular smooth muscle contractile machinery by suppressing the 1-adrenergic receptor agonist-mediated contraction while having no effect on membrane sensitivity of the endothelial or vascular smooth muscle cell layer. Furthermore, their mechanism of action seems to be accomplished through an endothelium-dependent pathway.