In vitro activity: Timolol Maleate is a beta-adrenergic antagonist similar in action to propranolol. The levo-isomer is the more active. Timolol has been proposed as an antihypertensive, antiarrhythmic, antiangina, and antiglaucoma agent. Similar to propranolol and nadolol, timolol is a non-selective, beta-adrenergic receptor antagonist. Timolol has a higher affinity for the beta 2-adrenoceptor than for the beta 1-adrenoceptor in human atrium. Timolol does not have significant intrinsic sympathomimetic, direct myocardial depressant, or local anesthetic (membrane-stabilizing) activity, but does possess a relatively high degree of lipid solubility. Timolol, when applied topically to the eye, has the action of reducing elevated, as well as normal, intraocular pressure, whether or not accompanied by glaucoma. Elevated intraocular pressure is a major risk factor in the pathogenesis of glaucomatous visual field loss and optic nerve damage. Like propranolol and nadolol, timolol competes with adrenergic neurotransmitters such as catecholamines for binding at β1-adrenergic receptors in the heart and vascular smooth muscle and β2-receptors in the bronchial and vascular smooth muscle. β1-receptor blockade results in a decrease in resting and exercise heart rate and cardiac output, a decrease in both systolic and diastolic blood pressure, and, possibly, a reduction in reflex orthostatic hypotension. β2-blockade results in an increase in peripheral vascular resistance. The exact mechanism whereby timolol reduces ocular pressure is still not known. The most likely action is by decreasing the secretion of aqueous humor.