In male rats, partial hemitransections but not 6-hydroxydopamine (6-OHDA)-induced lesions of the mesostriatal dopamine (DA) pathway produce after 7 days a marked and a modest increase of striatal putrescine and spermidine levels, respectively, on the lesioned side. Following chronic ganglioside GM1 treatment of partially hemitransected rats, an increase of striatal polyamine levels was observed also on the intact side. It is suggested that retrograde cell body changes produced by hemitransection may induce striatal ornithine decarboxylase activity and in this way increase striatal putrescine levels, favoring regenerative mechanisms. The increase of striatal polyamine levels by GM1 treatment on the intact side of both 6-OHDA and mechanically lesioned rats compared with intact unoperated rats may also reflect an increased synthesis of striatal polyamines.