A large class of (4,5-substituted-thiazol-2-yl)hydrazone derivatives (1–66) was synthesized in good yield (82–99%) and characterized by elemental analysis and 1 H NMR studies. The compounds were assayed for their in vitro human monoamine oxidase inhibitory activity and selectivity. Most of them showed IC50 values in the micromolar range. Our aim was to evaluate the importance of a bulky group at C2, C4, and C5 positions of the thiazole nucleus in modulating the biological activity of this scaffold.