A series of sulfonamides has been obtained by reaction of 4-isothiocyanatobenzenesulfonamide with amines, amino acids, and oligopeptides. The new thiourea derivatives showed strong affinities toward isozymes I, II, and IV of carbonic anhydrase (CA, EC 4.2.1.1). In vitro inhibitory power was good (in the low-nanomolar range) for the derivatives of beta-phenylserine and alpha-phenylglycine, for those incorporating hydroxy and mercapto amino acids (Ser, Thr, Cys, Met), hydrophobic amino acids (Val, Leu, Ile), aromatic amino acids (Phe, His, Trp, Tyr, DOPA), and dicarboxylic amino acids as well as di/tri/tetrapeptides among others. Such CA inhibitors displayed very good water solubility (in the range of 2-3%) mainly as sodium (carboxylate) salts, with pH values of the obtained solutions being 6.5-7.0. Some of these preparations (such as the derivatives of Ser, beta-Ph-Ser, Leu, Asn, etc.) strongly lowered intraocular pressure (IOP) when applied topically, directly into the normotensive/glaucomatous rabbit eye, as 2% water solutions. It is interesting to note that not all the powerful CA inhibitors designed in the present study showed topical IOP-lowering effects (such as, for instance, the Cys and Lys derivatives, devoid of such properties) whereas the Pro, Arg, and oligopeptidyl thiourea derivatives showed reduced efficacy when administered topically. This may be due to the very hydrophilic nature of some of these compounds, whereas inhibitors with balanced hydro- and liposolubility also showed optimal in vivo effects. The interesting pharmacological properties of this new type of CA inhibitors, correlated with the neutral pH of their solutions used in ophthalmologic applications, make them attractive candidates for developing novel antiglaucoma drugs devoid of major ocular side effects.