The synthesis, radiochemical analysis and biological characteristics of some 1,8-diamine-3,6-dithiaoctane derivatives labelled with Tc-99m are reported. Analysis by HPLC shows that most of the 99mTc-chelates are multicomponent. Furthermore, almost all 99mTc-complexes isolated by HPLC are lipophilic and stable in vitro. The biodistributions of the most lipophilic of these complexes were evaluated in mice. The N-morpholinylethyl and N,N'-bisalicylyl derivatives of 1,8-diamine-3,6-dithiaoctane yielded 99mTc-complexes which exhibit considerable uptake and retention in organs of interest, such as the heart and the brain.