We conducted a randomized, double-masked, dose-response study of the ocular hypotensive effect of the β-adrenergic blocker, levobunolol. A single drop of placebo or levobunolol (at concentrations of 0.03%, 0.3%, 0.6%, 1%, and 2%) was administered to one eye of each of 48 patients with ocular hypertension. The 0.3% and 0.6% concentrations decreased intraocular pressure significantly from baseline levels compared to placebo at one, two, and four hours after treatment. The 1% and 2% concentrations decreased intraocular pressure significantly from baseline compared to placebo at one, two, four, six, eight, and 12 hours after administration. No objective or subjective side effects were noted, and no substantial changes in visual acuity, pupil diameter, pulse rate, or blood pressure were recorded during the study. These results appear to justify long-term studies of levobunolol for the treatment of increased intraocular pressure.