A major problem in neural transplantation therapy is poor survival of grafted cells, which may be due to low cell viability prior to transplantation or scarce trophic factors available to the cells following transplantation. Recently, the delta enkephalin analogue [D-Ala(2),D-Leu(5)]-enkephalin (DADLE) has been demonstrated to protect against, as well as to reverse methamphetamine-induced loss of dopamine transporters. Here, we show that pretreatment with DADLE (0.0025, 0.005, 0.01 g/ml) dose-dependently enhanced cell viability of cultured primary rat fetal mesencephalic cells. In addition, DADLE administration in adult rats (4 mg/kg every 2 h, 4 injections, i.p.) prior to 6-hydroxydopamine lesions of the medial forebrain bundle, significantly reduced the severity of loss of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra 1 month post-lesion. This is the first report suggesting that DADLE can be used as a supplement factor for improving the cell viability of fetal mesencephalic cells and as a protective agent against neurotoxicity in a Parkinson's disease model.