Abstract The maximal depolarization of rabbit mesenteric veins by norepinephrine was significantly greater than that produced by serotonin, paralleling the previously reported inequality of the contractile effects of these drugs. It is suggested that the unequal effects of drugs on the ion-permeability of the membrane, if they also involved Ca, would account for their unequal efficiency of pharmacomechanical coupling. Very fast action potentials (rise time < 1 msec), of a type not described previously in vascular smooth muscle, have been recorded from a very few fibers of rabbit main pulmonary artery and in a coronary vein. The dose-dependent, graded depolarization of rabbit main pulmonary arteries, usually without action potentials, was verified by intracellular records. The effects of intracellular polarizing and depolarizing currents on action potentials of mesenteric veins were determined. Hyperpolarization increased the height and the rate of rise of action potentials, depolarization had the opposite effect. The differences between spike-generating and electrically gradedly responsive vascular smooth muscles (rate of relaxation of K, Ca-contractures, depression by Dibenamine, permeability to Ca) persisted after the depletion of endogenous catecholamine stores with reserpine. Procaine reversibly relaxed the K, Ca-contractures of both types of vascular smooth muscle.