The biotransformation of the tertiary amine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was studied in isolated rat hepatocytes, in order to assess the relative contributions of the metabolic reactions previously described in studies with subcellular preparations. The oxidative pathway which produces the toxic metabolite 1-methyl-4-phenylpyridinium ion (MPP+) via the generation of the 2,3-dihydropyridinium derivative, MPDP+, accounted for approximately 90% of MPTP metabolism by hepatocytes. Mitochondrial monoamine oxidase type B (MAO B) was specifically responsible for the initial step of this conversion. In the endoplasmic reticulum, cytochrome P-450 catalyzed the demethylation of MPTP to form PTP, while the flavin-containing monooxygenase (FMO) was responsible for the generation of MPTP N-oxide. No other metabolite was detected in our hepatocyte incubations under any of the experimental conditions used. After pretreatment with the specific MAO B inhibitor, deprenyl, the rates of production of the two microsomal metabolites were enhanced, but the overall rate of MPTP conversion decreased by more than 60%. On the other hand, no significant difference in the rate of MPTP metabolism was found after the inhibition of cytochrome P-450 by SKF 525-A or with the use of methimazole, a competitive substrate for FMO. The SKF 525-A and methimazole treatments selectively inhibited the formation of PTP and MPTP N-oxide, respectively, but had no significant effect on the rate and extent of MPTP toxicity. MPP+ was the only metabolite which accumulated within the cell compartment under all the experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)