About phentolamine mesylatea reversible nonselective alpha-adrenergic antagonist, imidazoline derivative, peripheral vasodilator ,antihypertensive.mechanism of action of phentolamine mesylatephentolamine is a quick & short acting non selective, competitive alpha adrenergic blocker and suppresses the presser actions of circulating catecholamines. it also blocks 5ht-receptors and potassium channels & causes release of histamine from mast cells. it produces dilatation of peripheral blood vessels, decrease peripheral resistance & blood pressure. it causes dilatation and relaxation of blood vessels and improves blood flow. when injected in to penis (intra-cavernosal) it increases blood flow to penis which results in erection.