In vitro activity: Tolazoline is a pulmonary vasodilator indicated used to decrease pulmonary vascular resistance (PVR) in persistent pulmonary hypertension of the newborn (PPHN). Tolazoline has moderate alpha-adrenergic blocking activity and has histamine agonist activity. Tolazoline usually reduces pulmonary arterial pressure and vascular resistance. Tolazoline is not as broadly effective as SNP against all spasmogens investigated. However, it may be effective in counteracting alpha-adrenoceptor-mediated vasospasm in human radial arteries.