Abstract In vitro studies have provided evidence that Cl − ion currents are important for activation of vascular smooth muscle contraction. The stilbene, 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS), disrupts Cl − metabolism by blocking Cl − channels and by inhibiting Cl − bicarbonate exchange. The aims of this study were to: (i) characterize the hemodynamic responses produced by DIDS in pentobarbital anesthetized rats, and (ii) examine vasoconstrictor responses to norepinephrine before and after administration of DIDS. DIDS (2.5–50 μmol/kg, 92.5 μmol/kg total dose, i.v.) produced dose-dependent but transient reductions in mean arterial blood pressure and in hindquarter, renal and mesenteric vascular resistances. Prior to the administration of DIDS, norepinephrine (1.0–5.0 μg/kg, i.v.) produced dose-dependent increases in mean arterial pressure, renal resistance and mesenteric resistance, but decreases in hindquarter resistance that were inversely related to dose. After administration of DIDS, the peak pressor responses produced by norepinephrine were either slightly diminished (1.0, 2.5 μg/kg) or unchanged (5.0 μg/kg). Peak norepinephrine-induced changes in hindquarter and renal vascular resistance were unaffected by DIDS, while increases in mesenteric resistance were augmented. The total norepinephrine-induced increases in mean arterial pressure (mm Hg×s) were markedly reduced by DIDS. These effects of DIDS on norepinephrine-induced responses were similar, but not identical to those of the voltage-sensitive Ca 2+ channel blocker, nifedipine (500 nmol/kg, i.v.). These findings suggest that DIDS may interfere with norepinephrine-induced depolarization of resistance arteries, thereby preventing activation of voltage-sensitive Ca 2+ channels.