Parkinson's disease (PD) is characterized by loss of dopaminergic (DA) neurons. Our previous studies revealed that DA neurons in Planarian Dugesia Japonica decreased after the amputation of their heads and the injection of 6-hydroxydopamine (6-OHDA), a DA neuron specific toxin, but regeneration of DA neurons was observed within 7 days. Recently, MAPK/ERK signaling was identified as a key regulator of planarian regeneration. In this study, we checked the effect of MEK inhibitors in the regeneration of DA neurons. Loss of DA neurons in Planarians of SSP strain were induced by the amputation of their heads and injection of 6-OHDA into their pharynxes. Regeneration of DA neurons were detected by immunohistochemical analysis with DjTH antibody at 7 days after amputation and at 5 days after 6-OHDA injection. Inhibition of MEK, upstream of ERK, by treatment of U0126 (10 µM) suppressed the regeneration of DA neurons. Furthermore, 5 days after 6-OHDA injection, phosphorylated-histone H3 (H3P)-positive stem cells were increased in head region, but MEK inhibition decreased H3P-positive cells. These data suggested that MEK/ERK signaling is a key regulator in the process of regeneration of DA neurons via proliferation of intrinsic stem cells.