The administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to mice causes a long-lasting depletion of striatal dopamine. The concomitant administration of a dopamine uptake blocker, cocaine, protected the mice from this MPTP-induced dopamine depletion. These observations are discussed in reference to the mechanism through which MPTP enters dopaminergic neurons as well as species differences in the sensitivity of the serotonergic system to the toxic effects of MPTP.