We evaluated the effect of topical ethacrynic acid on rabbit and monkey intraocular pressure. In a preliminary experiment, 100-mmol/L ethacrynic acid applied topically to Dutch-Belted rabbit eyes was associated with an 8-mm Hg lowering of intraocular pressure. However, corneal edema was severe, and the corneal epithelium sloughed off. To try to maintain the pressure-lowering effect but reduce the corneal side effects, we attempted to create an adduct of ethacrynic acid by utilizing ethacrynic acid's sulfhydryl reactivity. Ethacrynic acid was mixed with equimolar cysteine to bind the sulfhydryl-reactive sites on ethacrynic acid. The goal was to expose the cornea to adducted ethacrynic acid, which might then dissociate in the anterior chamber via a retro-Michael reaction. Intraocular pressure decreased 8.9 mm Hg (n = 40) with this treatment, and corneal edema was lessened (32 of 40 eyes had mild to no edema). However, we observed that when the eye was treated before ethacrynic acid-cysteine administration with topical acetylcysteine, the corneal side effects were reduced further and the intraocular pressure effect remained. In living cynomolgus monkeys receiving a single pretreatment drop of 75-mmol/L acetylcysteine followed by two drops of 130-mmol/L ethacrynic acid and 130-mmol/L cysteine, an intraocular pressure lowering of 9.9 mm Hg was observed (n = 7). However, in three of seven eyes corneal edema developed. Pretreatment with two drops of acetylcysteine eliminated the pressure-lowering effect but did not confer any added corneal protection. Our results indicate that topical ethacrynic acid-cysteine is effective in lowering the intraocular pressure of rabbits and cynomolgus monkeys and, when combined with acetylcysteine pretreatment, may offer the potential for a new topical therapeutic regimen for use in glaucoma.