The effective time course of a topical carbonic anhydrase inhibitor, MK-507, on intraocular pressure (IOP) was studied in 6 normal volunteers. As a baseline study, IOP was determined and aqueous protein concentration was determined by laser flare photometry every half hour for 5 hours with the instillation of a placebo. Then, an examination with the same time course was performed on the same subjects with the instillation of 1% MK-507 or oral administration of acetazolamide at a dose of 250 mg. MK-507 lowered IOP 30 minutes after the instillation, and showed a significant reduction of IOP at 1 hour. Aqueous protein concentration increased significantly 30 minutes after the MK-507 instillation. Acetazolamide lowered IOP 30 minutes after the administration, and showed a significant reduction of IOP at one hour. Aqueous protein concentration increased significantly one hour after the acetazolamide administration. Corneal thickness, anterior chamber depth, anterior chamber volume, and serum protein concentration were not significantly changed by the drug treatment. These results suggest that the inhibition of aqueous humor production causes the rapid reduction of IOP seen in the use of MK-507 as well as acetazolamide.