Aiming to contribute to the design of technetium imaging agents for estrogen receptor (ER) positive breast tumors, we have synthesized and evaluated the novel organometallic estradiol complexes (fac-[M(CO)(3)(κ(3)-10)](+) and fac-[M(CO)(3)(κ(3)-12) M=Re/(99m)Tc) using two different bifunctional tridentate ligands (4 and 8). The rhenium complexes (13 and 14) were fully characterized by IR, (1)H NMR, (13)C NMR, mass spectrometry and elemental analyses. The (99m)Tc complexes (15 and 16) were obtained with high radiochemical purity and exhibited high in vitro radiochemical stability. To get a first insight into the relevance of these complexes for targeting ER positive tumors, ER binding affinity assays and cellular internalization studies in an ER expressing cell line, MCF-7, have also been performed suggesting a non ER mediated uptake.