The intraocular pressure response to timolol ophthalmic solution, a beta-adrenergic blocking agent, was tested in two groups of open angle glaucoma patients using the 0,1% and the 0,5% solution. The 0.5% concentration gave a mean IOP reduction of 13 mm Hg in 39 glaucomatous eyes, corresponding to 46% of the pretreatment pressure level. The 0,1% concentration resulted in an average pressure decrease of 11,5 mm Hg in 22 glaucomatous eyes, corresponding to 40% of the pretreatment pressure levels. With both solutions a reduction of the first day's response was noted after repeated administration of timolol. In all patients tested timolol was tolerated well subjectively and objectively without any side effects. Timolol did not affect blood pressure or pulse rate. Tear production was not reduced after topical application of timolol. Pupillary diameter and facility of outflow were not changed significantly under therapy. It is concluded, that timolol acts primarily by reduction of aqueous inflow. The advantages of timolol as antiglaucomatous drug not affecting visual functions like miotics are discussed.