[The mesylate salt of a synthetic imidazoline with alpha-adrenergic antagonist activity. As a competitive alpha-adrenergic antagonist, phentolamine binds to alpha-1 and alpha-2 receptors, resulting in a decrease in peripheral vascular resistance and vasodilatation. This agent also may block 5-hydroxytryptamine (5-HT) receptors and stimulate release of histamine from mast cells. ( NCI )] (UMLS (NCI) C0733398) PHENTOLAMINE MESYLATE; PHENTOLAMINE MESYLATE PREPARATION =Organic Chemical; Pharmacologic Substance =2-[N-p'-Tolyl-N-(m'-hydroxyphenyl)aminomethyl]imidazoline; =PHENTOLAMINE MESYLATE 5