1-tert.-Butylamino-3-(2-cyclopentylphenoxy)-propan-2-ol (penbutolol, Hoe 893d) is a beta-adrenergic blocking agent about 4 times more active than propranolol in vivo and in vitro. In comparison to propranolol it is characterized by a longer lasting activity. The antihypertensive effect of penbutolol in spontaneously hypertonic rats is more than 5 times stronger than that of propranolol. Penbutolol reduces basal plasma renin activity in the same dose range as does propranolol but is about 3 times stronger with respect to isoproterenol-induced increase of PRA. Penbutolol is 5 times more potent than propranolol inhibiting isoproterenol-stimulated phosphorylase activity in the isolated heart. In reserpine pretreated rats, penbutolol has a moderate intrinsic sympathomimetic activity (ISA). Penbutolol shows less unspecific actions -- such as negative inotropy or calcium antagonism -- than propranolol. Characteristic parameters of lung function (compliance and resistance) are less affected by penbutolol than propranolol in spite of the fact that penbutolol has a stronger beta-adrenergic blocking effect.