Timolol maleate, a beta-adrenergic blocking agent, was used in a study of 20 adult patients with chronic open-angle glaucoma. This single-dose study demonstrated a dose response for this drug using concentrations of 0.1%, 0.25%, 0.5%, and 1.0%. Timolol, 0.5%, appears to give the maximal ocular hypotensive effect; however, each concentration of the drug produced an ocular hypotensive effect for at least 24 hours. No local or systemic side effects were discovered throughout the study. Timolol may be an effective, innocuous, once-a-day, topical agent for the treatment of glaucoma.