Tiprenolol (DU21445) has been shown by both in vitro and in vivo animal experiments, to possess strong beta-adrenergic blocking but a relatively less strong negative inotropic activity. The same report also demonstrated by its inhibiting influence on spontaneous mobility of rat uterus, an intrinsic beta-sympathomimetic activity. Tiprenolol reveals effects similar to propranolol. Tiprenolol was shown to have significantly less heart rate slowing effect at rest than propranolol. However, all other measurements failed to show any difference of statistical significance between the two drugs with respect to any negative inotropic or beta‐blocking activity. The administration of tiprenolol or propranolol depressed the arterial pressure and caused the deaths of some dogs in which a coronary artery had been ligated.