Timolol maleate is a nonselective beta-blocking drug used for the topical treatment of increased intraocular pressure in patients with chronic open angle glaucoma. In comparative studies it has had a somewhat greater ocular hypotensive effect than the sympathomimetic agent adrenaline (epinephrine), or lower to medium concentrations (up to 4%) of the miotic drug pilocarpine, usually lowering intraocular pressure by about 30 to 35% (absolute pressure reduction). Timolol has been well tolerated by most patients, producing fewer subjective complaints than the comparison drugs, and objective measurements of ophthalmic status have not revealed any marked changes. Although some patients have been treated for extended periods without serious adverse effects or loss of effectiveness, further published reports in larger numbers of patients treated over several years are needed to confirm the drug's apparent long term safety and continued efficacy. Nevertheless, on the basis of present evidence, timolol appears to represent an important advance in the topical treatment of glaucoma.