Nineteen cationic Tc-99m complexes, based upon four different ligand series, have been synthesized under "no carrier added" conditions and qualitatively evaluated as myocardial imaging agents in a dog model. Of these complexes, the four halogen derivatives of the diars series [diars = o-phenylenebis(dimethylarsine)] successfully concentrate in the myocardium. These Tc-99m diars complexes are shown by thin-layer chromatography to be identical to the Tc-99 analogs that have been fully characterized as trans octahedral complexes of Tc(III) by classical chemical techniques. Tissue distribution studies in rats show a definite difference in the biodistributions of [99mTc(diars)2Cl2]+ and [99mTc(diars)2Br2]+, despite the similar size, shape, charge, and lipophilicity of these two complexes. A tissue distribution study in resting beagle dogs shows that [99mTc(diars)2Br2]+ accumulates in the normal myocardium about half as well as TI-201 (0.022 compared with 0.038% dose/g).