The relationship between oxidative polymorphisms and the cause of Parkinson's disease is controversial. The drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which induces parkinsonism in humans and in some animal models, is metabolized by cytochrome P450 db1 isozyme (the same enzymatic system implicated in 4-hydroxylation of debrisoquine). In this study, we treated females of three rat species, which differ in their ability to hydroxylate debrisoquine, with MPTP (three doses of 30 mg/kg s.c. at 12-h intervals), and we measured their motor activity and brain monoamine levels. Female dark-adapted rats (poor metabolizers of debrisoquine) showed a more pronounced and more maintained reduction of their motor activity after treatment with MPTP. MPTP-treated, dark-adapted rats also had a depletion of noradrenaline in the diencephalon and a depletion of dopamine and serotonine and their respective metabolites in the limbic system when compared with the other two species. These results suggest that oxidative polymorphism of debrisoquine plays a role in the acute effects of MPTP.