Unilateral intranigral injections of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MPP+ (1-methyl-4-phenylpyridine) were given to young rats and unilateral intranigral injections of MPTP were given to old rats. MPTP in old rats and MPP+ in young rats induced ipsiversive circling for at least one week after injection and contraversive circling after the systemic administration of apomorphine; the number of D-2 receptors (Bmax) in the striatum of the injected hemisphere increased compared with that of control rats. MPTP in young rats induced only short-lasting ipsiversive circling and no contraversive circling after apomorphine; the number of striatal D-2 receptors did not increase. These results suggest that the neurotoxicity of MPTP is age-dependent in the rat, and that MPTP has neurotoxic effects on the nigrostriatal dopaminergic system in old rats and induces dopamine receptor supersensitivity in the denervated striatum.