Two novel 99mTc-(SNS/S) complexes: a mono-ester compound carrying an ethyl ester group on the tridentate ligand, 99mTcO[C2H5OOCCH2N(CH 2CH2S)2][SC6H4CH 3], 3, and a diester compound, carrying a second ethyl ester group on the monodentate ligand, 99mTcO[C2H5OOCCH2N(CH 2CH2S)2][SC6H4COOC 2H5], 4, were synthesized. The corresponding oxorhenium(V) complexes, 1 and 2 were also synthesized. Enzymatic hydrolysis demonstrated that 3 remains intact after 10 min incubation while 4 is totally converted to an unidentified hydrophilic complex. Tissue distribution data in mice revealed that both complexes, 3 and 4, exhibit significant initial brain uptake (1.42 and 1.01% of injected dose at 5 minutes post injection respectively) and fast blood clearance.