A phthalazine derivative with antihypertensive effects. Hydralazine exerts its vasodilatory effects through modification of the contractile state of arterial vascular smooth muscle by altering intracellular calcium release, and interfering with smooth muscle cell calcium influx. This agent also causes inhibition of phosphorylation of myosin protein or chelation of trace metals required for smooth muscle contraction, thereby resulting in an increase in heart rate, stroke volume and cardiac output.