Abstract—Increased expression of phosphoinositide 3-kinase (PI3-kinase) mediates elevated tone in the aorta from hypertensive deoxycorticosterone acetate (DOCA)-salt rats. In this article, we hypothesized that (1) alterations observed with respect to PI3-kinase observed in the aorta would also occur in mesenteric resistance arteries responsible for determining total peripheral resistance (TPR) and (2) p110&dgr; activity was increased and localized to vascular smooth muscle cells (VSMCs), and was responsible for the increase in spontaneous tone in aortae from DOCA-salt rats. Mesenteric resistance arteries and aorta were isolated from DOCA-salt (190±3 mm Hg) and sham (121±2 mm Hg) rats. Myograph experiments revealed LY294002 (20 &mgr;mol/L), a PI3-kinase inhibitor, significantly decreased tone in mesenteric resistance arteries from DOCA-salt rats as compared with sham (−49±12 mg versus −10±7 mg). Western analyses of resistance artery protein homogenate revealed p85&agr; and p110&dgr; subunit protein, with significantly elevated levels of p110&dgr; protein in the DOCA-salt compared with sham rats (0.30±0.07 versus 0.16±0.04% smooth muscle alpha-actin arbitrary units). Immunohistochemistry revealed p110&dgr;-specific staining in VSMCs, with more intense staining in aortae from DOCA-salt rats. Compared with aortae from sham, p110&dgr;-associated PI3-kinase activity was increased in DOCA-salt (158% of sham) and likely responsible for spontaneous tone because the p110&dgr; specific inhibitor IC87114 decreased spontaneous tone in a concentration-dependent manner. Collectively, these data further implicate the p110&dgr; isoform of PI3-kinase in arterial hyperresponsiveness in hypertension at the level of both large and small arteries.