Endothelin (ET-1) has many activities in various tissues and organs. Even in the vascular system, ET-1 elicits vasodilation as well as vasoconstriction. These diverse responses induced by ET-1 are ascribed to the balance between ET-1 and ET-3 effects that are mediated by (a) at least two types of calcium channel activated by ET, (b) three distinct prototypes of ET-receptor subtypes, (c) the diversity of the intracellular signal transduction system, especially cyclooxygenase products, and (d) modulation of hormonal and neuronal factors.