Two new ligand systems for complexation with 99mTc were prepared. The two analogs of bisaminoethanethiol (BAT): N,N'-bis(2-methyl-2-mercaptopropyl)-2,2-dimethylpropylenediamin e (PAT-HM) and N,N'-bis[2-(2-ethyl-1-mercaptopropyl)] ethylenediamine (TMR), form neutral and lipid soluble complexes with 99mTc that readily penetrate the blood-brain barrier following i.v. injection into rats. Although the 99mTc chelates do not display the prolonged brain retention required for use in single photon emission computed tomographic imaging studies, the fact that each ligand forms a neutral and lipid-soluble complex of high chemical stability when coordinated with 99mTc warrants further investigation to increase the site- and organ-specificity of these agents.