Brimonidine, a highly selective α<sub>2</sub>-adrenoceptor agonist, was studied to determine its ocular hypotensive effect and side effects in patients with elevated intraocular pressure (IOP) while on continuing therapy with timolol. This was a prospective, randomized, placebo-controlled study in 15 patients with primary open-angle or pseudoexfoliation glaucoma on therapy receiving timolol 0.5% twice daily, with IOP greater than or equal to 22 mm Hg in one eye. IOP measurements, blood pressure and pulse rate were assessed on 2 days at a baseline and 1, 2, 4, 6 and 8 h later. A single drop of brimonidine 0.2% or placebo was added to treatment with timolol. The reductions in IOP at all time intervals observed with brimonidine + timolol were significantly greater than those with timolol + placebo. The maximum mean net decrease in IOP was 19.23 ± 10.60% at 4 h. Statistically significant decreases in systemic blood pressure and pulse rate without clinical symptoms were observed in the group receiving brimonidine + timolol. This study suggests that a combination of brimonidine and timolol may have potential in the treatment of glaucoma. Further clinical trials with brimonidine are indicated to assess its further role as adjunctive agent.