Reasons for performing study: Only few drugs with limited efficacy are available for topical treatment of equine glaucoma. Objective: To evaluate the effect of topical administration of 1% brinzolamide on intraocular pressure (IOP) in clinically normal horses. Methods: Healthy mature horses (n = 20) with normal ocular findings, were studied. The IOP was measured 5 times daily (7000, 1100, 1500, 1900 and 2300 h) over 10 days, On Days 1 and 2, baseline values were established. On Days 3-5 one eye of each horse was treated with one drop of 1% brinzolamide every 24 h immediately following the 0700 h measurement. On Days 6-8 the same eye was treated with 1% brinzolamide every 12 h (0700 and 1900 h). Measurements on Days 9 and 10 documented the return of IOP to baseline values. Statistical analysis of the data was performed. Results: In the treated eye a significant decrease in IOP compared to baseline values was noted during both the 24 and 12 h dosing periods (P<0.001). During the once-daily treatment protocol an IOP reduction of 3.1 +/- 1.3 mmHg (14%) from baseline was recorded. During the twice-daily protocol a total IOP reduction of 5.0 +/- 1.5 mmHg (21%) was achieved. Conclusion: Intraocular pressure was significantly decreased by 1% brinzolamide in a once-daily and a twice-daily treatment protocol in normotensive eyes. These findings suggest that brinzolamide might also be effective in horses with an elevated IOP. Potential relevance: This drug may be useful for treatment of equine glaucoma.