The stereoisomer form of timolol used in the treatment of glaucoma is l-timolol. Although d-timolol is a less potent beta-adrenergic receptor blocker than l-timolol, several laboratory studies have found that d-timolol has ocular hypotensive effects. Thus, d-timolol may be a useful therapeutic agent for glaucoma that has fewer systemic side effects than l-timolol. We conducted a randomized, double-masked, single-drop study of the effects of d-timolol and placebo on intraocular pressure in 34 patients with ocular hypertension. d-Timolol significantly lowered intraocular pressure for the six-hour duration of the study. No patients receiving the drug reported subjective side effects. There was no change in visual acuity, pupil size, or results of external ocular or slit-lamp examinations during the study. No changes in pulse rate or blood pressure were attributable to the drug.