The antipsychotic drugs currently used to treat schizophrenia can be divided into two distinct classes, typical and atypical antipsychotics. Many drug molecules are enzyme inhibitors that bind reversibly or irreversibly to their target through intermolecular interactions. That's why enzyme inhibition studies are an important issue for drug design and biochemical applications. In this study, in vitro inhibition effect of some antipsychotic drugs on the purified carbonic anhydrase (CA) I and II isoenzymes were investigated by using CO2 as a substrate. CA I and II were purified from human erythrocytes by a simple one step procedure using Sepharose 4B-L-tyrosine-sulfonamide affinity column. The results showed that all the drugs inhibited the cytosolic carbonic anhydrases enzyme activity in a concentration-dependent fashion. Among the studied drugs, aripiprazole and pramipexole were found to be the most active one for hCA I (IC50: 3.64 and 5.37 mu M) and hCA II (IC50: 4.16 and 4.81 mu M) activity, respectively.