In human liver microsomal preparations the neurotoxic chemical N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and several of its analogs competitively inhibited bufuralol 1'-hydroxylase activity of cytochrome P450bufI. This enzyme is the target of the common genetic polymorphism of drug oxidation known as debrisoquine polymorphism. Bufuralol 1'-hydroxylase activity was detectable in rat brain tissue. The activity was inhibited by antisera raised against a rat liver cytochrome P450 called P450db1. Immunoblotting experiments revealed the presence of a protein in rat and human brain microsomes with the same electrophoretic properties as the liver enzyme. These data suggest that P450bufI may be involved in the metabolism and neurotoxicity of MPTP.