45 patients with ocular hypertension or open-angle glaucoma were tested to see if, and to what extent timolol and three other different parasympathicomimetics (pilocarpine 2%, carbachol 1.5%, aceclydine 2%) produced an supplementary reduction of the intraocular pressure. An equally marked, additional drop in eye pressure was achieved when patients who had been given timolol over an extended period of time, were additionally treated with one dose of pilocarpine 2% or carbachol 1.5%. Aceclydine 2% produced no statistically significant additional effect. Patients who had only been treated with pilocarpine 2% over an extended period of time and then were given one dose of timolol 0.25% produced a drop in eye presure more than double that of those patients who had been primarily treated with timolol and then received pilocarpine or carbachol. These results will be discussed on the basis of the mode of action of the applied medications.