N-Methylacetazolamide was shown to be active topically in reducing intraocular pressure (IOP) to a small but statistically significant level in the normotensive rabbit eye. In vivo experiments with N-methylacetazolamide suggest that ocular metabolism to acetazolamide was responsible for the observed topical activity. Examination of initial rate kinetics of carbonic anhydrase catalyzed p-nitrophenyl acetate hydrolysis showed that N-methylacetazolamide was a competitive inhibitor, in contrast to noncompetitive inhibition seen with acetazolamide and other primary sulfonamide inhibitors. N-Substituted and unsubstituted 4-chlorobenzene- and 4-nitrobenzenesulfonamides were also synthesized, and their biochemical characteristics and in vivo ability to lower IOP when applied topically were determined. The primary sulfonamides were reversible noncompetitive inhibitors of carbonic anhydrase, with no effect on IOP after topical administration. 4-Nitrobenzene- and 4-chlorobenzenesulfonamides containing both N-hydroxy and N-methyl substituents were model irreversible inhibitors of carbonic anhydrase and exhibited a trend toward topical activity in reducing IOP in normotensive rabbit eyes. Therefore, this paper describes the synthesis and characterization of two types of carbonic anhydrase inhibitors; the N-methyl-substituted sulfonamides are reversible competitive inhibitors of carbonic anhydrase, while the N-hydroxy-N-methyl-substituted sulfonamides are irreversible inhibitors.