In a search of coordination molecules suitable to the fac-(Tc-99m(Co)(3)}(+) core as a synthon for Tc-99m-radiopharmaceuticals, nonradioactive rhenium complexes of two macrocyclic triamine compounds with different chelate ring structures, 1,4,7-triazacyclononane (9N3) and 1,5,9-triazacyclododecane (12N3), were synthesized and characterized. Tc-99m-labeled 9N3 and 12N3 compounds were also prepared using [Tc-99m-(OH2)(3)(CO)(3)](+) and were characterized by both in vitro and in vivo studies. 9N3 produced a single rhenium complex, Whereas 12N3 generated two major complexes. The crystallographic data and infrared absorption wavenumber assigned to the C-O stretch suggested that the coordination geometry of 9N3 would be more suitable to fac-{Re(CO)(3)}(+) than that of 12N3. In contrast, both 9N3 and 12N3 provided a single Tc-99m-labeled compound, However Tc-99m-labeled 9N3 exhibited higher stability than Tc-99m-labeled 12N3 in rat plasma and in the presence of histidine at an elevated temperature. In biodistribution studies, both Tc-99m-labeled compounds did not show any specific accumulation of radioactivity in any organs except for the excretory organs such as the liver and kidney. These findings showed that 9N3 would constitute a macrocyclic chelating molecule of choice to prepare Tc-99m radiopharmaceuticals using a fac-{Tc-99m(CO)(3)}(+) core.