Several new 99mTc aminodithiols were prepared and evaluated comparatively in experimental animals. The ligands were diamine, triamine or tetramine dithiols. Substituents were either attached on one of the nitrogens or introduced in between the two nitrogens of diamino dithiol (DADT) backbone. 99mTc-derivatives prepared by coupling DADT to secondary amines via ethylene group showed in mice high initial brain uptake and significant retention in brain tissue. These preparations were mixtures of more than one 99mTc-complex differing in brain uptake and clearance from the brain. The highest brain retention (brain to blood ratio 2.53, 15 min p.i.) was achieved with the 99mTc-complex prepared by coupling DADT with ethylene pyrrolidine. Lengthening the chain between the nitrogens of DADT moiety by introducing methyl or amino alkyl groups resulted in 99mTc-complexes with poor brain accumulation.