Our previous studies demonstrate that chronic insulin administration exacerbates hypertension in spontaneously hypertensive rats (SHR). In the present study, we tested the hypothesis that the pressor effect of insulin in SHR is medicated by sympathetic nervous system overactivity. Male SHR (7 weeks old) were given daily subcutaneous injection of insulin or vehicle for 3 days, after which each rat received an intravenous infusion of the peripheral ganglionic blocker hexamethonium. Two days later, in a second experiment, the infusion protocol was repeated with the α 2-adrenoceptor agonist clonidine, which more selectively inhibits sympathetic (as compared with parasympathetic) nervous system activity. Insulin treatment for 3 days caused a significant increase in mean arterial pressure (MAP; 164±2 mm Hg vs. saline control 148±3 mm Hg), but ganglionic blockade with hexamethonium eliminated the difference in blood pressure (BP) between the insulin-treated and control SHR. Infusion of clonidine significantly reduced MAP in the insulin-treated group to the level of the untreated control SHR, but the infusion did not reduce MAP in the latter group. In a second group of rats, acute administration of prazosin also eliminated the difference in MAP between insulin-treated and control SHR. We conclude that in SHR the sympathetic nervous system contributes importantly to the pressor effect of insulin administration and that this effect may be mediated by the central nervous system.