One drop of 0.5% timolol maleate instilled into the normal eyes of four subjects produced a marked reduction of intraocular pressure that was maximal at 3 hours but not significant after 12 hours. Instillation of one drop a day for 30 days produced a reliable, maintained reduction of intraocular pressure in the treated eye. The pupil cycle time in both eyes was measured at least twice daily for 1 week before timolol treatment to assess the reproducibility of the test and then twice daily for the 30 days of treatment with timolol. The drug had no significant effect on the pupil cycle time. Measuring the pupil cycle time should afford the ophthalmologist a useful method of assessing the progress of glaucomatous damage to the optic nerve while the patient is undergoing treatment with timolol, an assessment that was not possible when patients were treated with pilocarpine.