Pyrazole carboxylic acid amides of 5-amino-1,3,4-thiadiazole-2-sulfonamide 1 (inhibitor 1) were synthesized from 4-benzoyl-1-(4-nitrophenyl)-5-phenyl-1H-pyrazole-3-carbonyl chloride and 4-benzoyl-1-(3-nitrophenyl)-5-phenyl-1H-pyrazole-3-carbonyl chloride compounds. Human carbonic anhydrase isoenzymes (hCA-I and hCA-II) were purified from erythrocyte cells by the affinity chromatography. The inhibitory effects of inhibitor 1, acetazolamide (AAZ), and of 16 newly synthesized amides (8-11, 12a-f, 13a-c, 14a-b, and 15) on hydratase and esterase activities of these isoenzymes have been studied in vitro. The average IC(50) values of the new compounds (8-11, 12a-f, 13a-c, 14a-b, and 15) for hydratase activity ranged from 3.25 to 4.75 microM for hCA-I and from 0.055 to 2.6 microM for hCA-II. The mean IC(50) values of the same inhibitors for esterase activity were in the range of 2.7-6.6 microM for hCA-I (with the exception of inhibitor 10, which did not inhibit the esterase activity of hCA-I) and of 0.013-4.2 microM for hCA-II. The K(i) values for new compounds (8-11, 12a-f, 13a-c, 14a-b, and 15) were observed well below that of the parent compound inhibitor 1 and were also comparable to that of AAZ under the same experimental conditions. The comparison of newly synthesized amides to inhibitor 1 and to AAZ indicated that the new derivatives preferentially inhibit hCA-II and are more potent inhibitors of hCA-II than the parent inhibitor 1 and AAZ.