Growth/differentiation factor 5 (GDF5) is a neurotrophin which protects the rat nigrostriatal dopaminergic pathway fr om 6-hydroxy-dopamine-induced damage. Here we used amphetamine-induced rotational testing, high-performance liquid chromatography and immunocytochemistry to investigate the minimum effective dose of GDF5. Wt also compared the effectiveness of injecting GDF5 into either the substantia nigra pars compacta (SNpc), the lateral ventricle (LV) or the striatum (or combinations of these sites). (C) 1999 Elsevier Science B.V. All rights reserved.