Complexation of ligands containing an N3S donor set has been affected with [99mTc]. These are part of a ligand series of analogous structures which exhibit similar chemistry and potentially interesting biology. The complexes which have been characterized with [99Tc] as [TcOL] are neutral and lipophilic and their biological behaviour has been assessed in rats. After HPLC purification of the no-carrier added preparation, brain uptake of the tracers was greater than 1% at 15 min p.i. Muscle activity was significant with slow blood clearance.