In rabbits, l-timolol [26839-75-8] and (±)-oxprenolol [6452-71-7] antagonized (±)-isoproterenol [7683-59-2]-induced elevation of cAMP [60-92-4] levels in the aqueous humor and decreases in intraocular pressure, whereas (±)-alprenolol [13655-52-2] and (±)-practolol [6673-35-4] were inactive in both respects.Apparently, elevation of cAMP levels by isoproterenol and its ability to lower intraocular pressure are interrelated.The lack of a relationship between the ability of the nonselective β-blocking agents timolol, alprenolol, oxprenolol, and (±)-propranolol [525-66-6] and the cardioselective (β1-) agent practolol to lower intraocular pressure and to antagonize isoproterenol further suggests that the receptors at which β-adrenergic blocking agents act in the rabbit eye to lower intraocular pressure may have characteristics or properties different from classical β-adrenergic receptors.