Concentrations of droperidol which caused a shift to the right of the dose-response curve to noradrenaline in the pulmonary artery and the saphenous vein of the dog did not affect myogenic activation by K+; they did not inhibit spontaneous activity of portal-mesenteric veins. Droperidol inhibited the contractile response to nerve stimulation, but did not affect the evoked release of 3H-noradrenaline. These experiments indicate that the vasodilator properties of smaller doses of droperidol are a result of its ability to block alpha-adrenergic receptors.