) found that endothelin (ET) stimulated O 2 Ϫ production in sympathetic ganglion neurons in vitro by activating ETB receptors. The objective of the present study was to determine whether activation of ETB receptors in vivo elevates O 2 Ϫ levels in sympathetic ganglia. Because ET B receptor activation increases blood pressure, we also sought to determine whether alteration in O 2 Ϫ levels was a direct effect of ET B receptor activation on sympathetic ganglia or an indirect consequence of hypertension. Male Sprague-Dawley rats received intravenous infusions of either the specific ET B receptor agonist sarafotoxin 6c (S6c; 5 pmol ⅐ kg Ϫ1 ⅐ min Ϫ1 ) or isotonic saline at 0.01 ml/min (control) for 120 min. To measure O 2 Ϫ levels, we removed the inferior mesenteric ganglion immediately after infusion and stained it with dihydroethidine (DHE). Mean arterial pressure increased 26.6 Ϯ 1.7 mmHg in the S6c-treated rats and 3.65 Ϯ 6 mmHg in control rats. Measurements of average pixel intensity revealed that the DHE fluorescence in ganglionic neurons and surrounding glial cells were 96.7% and 160% greater in S6c-treated than in control rats, respectively. To evaluate the effect of elevated blood pressure on O 2 Ϫ production, a separate group of rats received phenylephrine (PE; 10 g ⅐ kg Ϫ1 ⅐ min Ϫ1 iv) for 2 h. MAP increased 31 Ϯ 1.2 mmHg in PE-infused rats. The DHE fluorescence intensity in ganglia of PEinfused rats was significantly greater than that of control rats, 137.7% in neurons and 104.6% in glia but significantly lower than in ganglia from S6c rats. We conclude that ETB receptor activation in vivo significantly enhances O 2 Ϫ levels in sympathetic ganglia, due to both pressor effects and direct stimulation of ET B receptors in ganglion cells. hypertension; sarafotoxin 6c; sympathetic nervous activity; oxidative stress; reactive oxygen species HYPERTENSION CAUSED BY NUMEROUS genetic and neurohumoral factors is associated with higher amounts of reactive oxygen species (ROS) in blood vessels, brain, and kidneys; examples include ANG II-mediated hypertension, deoxycorticosterone acetate (DOCA)-salt hypertension, mineralocorticoid hypertension, aortic banding-induced hypertension, renovascular hypertension, and endothelin-induced hypertension (1, 3, 18 -20, 23, 28, 36, 42). The best characterized ROS in tissues of hypertensive individuals is superoxide anion (O 2 Ϫ ). Reduction in O 2 Ϫ formation can lower blood pressure in some experimental models of hypertension (1,9,14,32,38), suggesting that increased production of ROS is an etiologic factor in hypertension. O 2 Ϫ can increase blood pressure by several mechanisms. In the vasculature, O 2 Ϫ causes vasoconstriction, in part by inducing endothelial cell dysfunction (5). Increased O 2 Ϫ in the kidney is associated with enhanced tubular reabsorption of sodium and water (27,33). In key brain regions, increased O 2 Ϫ leads to increased sympathetic nervous system activity (SNA) (7,(51)(52)(53). The focus of the work to be reported here, however, is on the peripheral sympa...