A series of hydrazinecarbothioamide derivatives incorporating ethyl, phenyl, tolyl, benzyl, and allyl moieties were prepared and tested as possible inhibitors of three members of the pH regulatory enzyme family, carbonic anhydrase (CA; EC 4.2.1.1). The inhibitory and activatory potencies of the compounds against the cytosolic human isoforms hCA I and hCA II and the transmembrane, tumor-associated hCA IX were analyzed by a hydrase assay with CO2 as substrate, and the inhibition constants (KI) were calculated. Most compounds investigated here exhibited nanomolar or low micromolar inhibition constants against the three isoenzymes. KI values were in the range of 34.1-871 nM for hCA I and compounds 5-10 showed interesting activation of the hCA II with KA value of 0.81-12.5 μM. Compounds 11-16 exhibited moderate inhibition effects on hCA IX in the range of 0.317-1.245 μM but they were less effective for hCA II. Tested compounds were also investigated using in silico applications at the binding pockets of these three targets. The different mechanisms of inhibition by these tested compounds as compared to sulfonamides, and their diverse inhibition profile for these mammalian isozymes, makes this class of derivatives of great interest for the design of novel CA inhibitors.