Abstract: Radiolabeling of biologically active molecules with the fac¹⁸⁸Re(CO)₃(H₂O)₃⁺ unit has been of primary interest in recent years. Therefore, the tridentate ligands L₁~L₄ (L₁ =histidine, L₂ = nitrilotriacetic, L₃ = 2-picolylamine-N,N-diacetic acid, L₄ = bis(2-pyridymethy)amine)were evaluated in radiochemical reactions with the fac¹⁸⁸Re(CO)₃(H₂O)₃⁺. These reactions yielded the radioactive building blocks fac¹⁸⁸Re(CO)₃L (L = L₁~L₄), which were identified by HPLC. Tricarbonyl complexes, with lg P values ranging from -2.23 to 2.18, were obtained in yields higher than 90% using ligand concentrations within the 1×10^-5~1×10^-4 mol/L range. Competition studies with cysteine and histidine revealed high stability for all of these radioactive complexes, and biodistribution studies in mice indicated a fast rate of blood clearance and high rate of total radioactivity excretion occurring primarily through the renalurinary pathway. In summary, complexes (L₁~L₄) are potent chelators for the future functionalization of biomolecules.