Among the human neuropeptide Y (NPY) receptors, the Y5 receptor (Y5R) represents a potential therapeutic target for the treatment of obesity and stress-related diseases. As the Y5R is expressed in various malignant tumors, it is also considered a potential target for cancer diagnosis and therapy. Although numerous non-peptidic Y5R antagonists with high binding affinity have been reported, only labeled peptidic agonists are used in routine Y5R competition binding assays, which are needed for the development and characterization of Y5R ligands. In the present study, three radiolabeled derivatives of the high-affinity Y5R antagonist CGP71683A (1) were synthesized ([3H]26, [3H]32, [3H]42) and studied in saturation binding assays using adherent or suspended HEC-1B-hY5R cells or membrane preparations thereof. Their Kd values (1.5–13 nM) were consistent with the Ki values (1.3–11 nM) of the non-labeled analogs. However, all synthesized radioligands exhibited very high non-specific binding, primarily caused by binding of the radioligands to cellular membranes, which limits their use as tool compounds in radiochemical Y5R binding assays. Nevertheless, a competition binding assay with [3H]32 and the endogenous Y5R agonist hNPY afforded a Ki that was in agreement with literature data.