Unilateral injection of MPTP into the pars compacta of the substantia nigra in rats induced contraversive turning immediately after the injection. Contraversive turning decreased, and reversed its direction after about 30 min. Ipsiversive turning was still present 24 h after the injection of MPTP. These results suggest that MPTP has an initial stimulatory effect on dopaminergic neurons followed by a depression of the activity of these cells.