Little is known about the function and regulation of the Na+-K+-2Cl−cotransporter NKCC1 in vascular smooth muscle. The activity of NKCC1 was measured as the bumetanide-sensitive efflux of86Rb+from intact smooth muscle of the rat aorta. Hypertonic shrinkage (440 mosmol/kgH2O) rapidly doubled cotransporter activity, consistent with its volume-regulatory function. NKCC1 was also acutely activated by the vasoconstrictors ANG II (52%), phenylephrine (50%), endothelin (53%), and 30 mM KCl (54%). Both nitric oxide and nitroprusside inhibited basal NKCC1 activity (39 and 34%, respectively), and nitroprusside completely reversed the stimulation by phenylephrine. The phosphorylation of NKCC1 was increased by hypertonic shrinkage, phenylephrine, and KCl and was reduced by nitroprusside. The inhibition of NKCC1 significantly reduced the contraction of rat aorta induced by phenylephrine (63% at 10 nM, 26% at 30 nM) but not by KCl. We conclude that the Na+-K+-2Cl−cotransporter in vascular smooth muscle is reciprocally regulated by vasoconstrictors and nitrovasodilators and contributes to smooth muscle contraction, indicating that alterations in NKCC1 could influence vascular smooth muscle tone in vivo.