In a study lasting 16 months, Timolol was tested on 57 eyes of a group of selected patients with open-angle glaucoma. Patients whose IOP could not be stabilized with the 0.25% solution were treated with 0.5% drops twice a day. In five separate cases, where regulation was inadequate, pilocarpine 2% twice a day was prescribed in addition. At the end of 16 months' treatment pressure had been lowered to a statistically highly significant degree in 66% of the patients. While the degree of regulation in the remainder was not statistically sifnificant, most of them were adequately stabilized. Further, following stabilization with Timolol, 42% of the patients were subjected to tolerance tests under constant conditions (darkness test, water test, coffee and tea tolerance) and their 6.00 a.m.values were measured. While good pressure levels were recorded in the tolerance tests, the early-morning IOP rose to critical threshold levels in some patients. However, during the entire period of observation visual fields and optic disk findings remained unaltered in all cases. Only a few cases were subjective and/or objective side-effects seen. Ten percent of those patients who had been treated previously required new glasses (+0,5 to +1 D over the previous correction).